Methods of designing a surgical device

ABSTRACT

Provided is a method for designing a patient-specific surgical device for performing knee surgery, which includes determining a first alignment axis and/or a second alignment axis from a knee joint in extension and/or flexion respectively and subsequently designing said surgical device based on the first and/or second alignment axes. Also provided is a method of manufacturing a patient-specific surgical device designed by the aforementioned method. A patient-specific surgical device designed and/or manufactured by the above methods and a method of performing knee surgery with said patient-specific surgical device is further provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/574,950, filed on Nov. 17, 2017, which application is a United StatesNational Phase filing of International Application No.PCT/AU2016/050409, filed May 26, 2016, which claims priority to and thefull benefit of Australian Provisional Application Serial No.2015903054, filed Jul. 31, 2015 and Australian Provisional ApplicationSerial No. 2015901930, filed May 26, 2015, and titled “METHOD OFDESIGNING A SURGICAL DEVICE”, the entire contents of each priorapplication are incorporated herein by reference.

FIELD

THIS INVENTION described herein relates generally to a method ofdesigning a device for use in surgery. In particular, the invention isdirected to a method of designing a customized device for use in kneesurgery and, in particular, total knee replacement that achieves optimalsoft tissue balancing and joint line restoration in a subject in bothflexion and extension.

BACKGROUND

Total knee arthroplasty (TKA) or total knee replacement (TKR) is asurgical technique in which a surgical implant is inserted to replacethe knee joint. Typically, TKA/TKR is a successful and long lastingsurgical procedure but 10-20% of patients express some dissatisfactionregarding the outcome of the procedure or suffer from persistent pain.The skilled artisan would recognize that to achieve a good clinicaloutcome with TKR, correct implant sizing and alignment, optimal softtissue balancing, equal flexion and extension gaps and joint linerestoration are required.

Standard or universal cutting or resection blocks may be used to preparethe distal portion of a femur and/or the proximal portion of a tibia intotal knee arthroplasties (TKA's). Such cutting blocks are generallymounted to the femur and tibia by an adjustable instrument referenced byand extending from an intramedullary or extramedullary rod and thensubsequently directly to the resected surface of the femur and tibia.The instruments are made adjustable so that they may be applieduniversally to patients. While there are many benefits to adjustableinstruments, there also exist many disadvantages. Some disadvantagesinclude increased overhead, bulky kits and containers, unnecessary orredundant instruments, large numbers of trials and different sizes,increased sterilization time between surgeries, and increased financialrisks to orthopaedic manufacturers to maintain largely unused inventory.Furthermore, the universal instruments do not consistently reproduce thejoint line and balance throughout the range of motion of the knee thatis both within normal limits and specific to that patient.

More recently, the surgical process of TKA/TKR has been streamlined bythe introduction of customised resection guides (also referred to aspatient-specific or patient-matched instrumentation) so as to avoid oneor more of the aforementioned disadvantages of standard resectioninstruments. They do fail however to consistently reproduce correctligament balance throughout knee range of motion. Such customizedresection guides are typically formed using patient-specific anatomicaldata derived from, for example, a CT or MRI scan of a patient's kneejoint.

Prior art customised resection members have generally been designed tomake bony resections that either: (i) match an approximation of apatient's pre-arthritic knee anatomy; or (ii) are aligned with themechanical axis of the patient's knee. With regard to the pre-arthriticknee anatomy-matched resection members of (i), these devices aredesigned to place the cuts so that the knee is returned to its formeranatomical alignment prior to any loss of articular cartilage. Suchdevices are based solely on partial scans of the knee joint that areused to approximate mechanical axes by extrapolating a fixed number ofdegrees from the small anatomical axis portion visible thereon.

Conversely, mechanical axis-aligned customised resection members measurethe pre-operative mechanical axes based on scans typically of the entirelimb including the ipsilateral hip and ankle joint. Such resectionmembers aim to restore the alignment of a patient's limb to thismechanical axis at the end of the procedure. To this end, the devicesare orientated using current surgical principles and are typicallyplaced in relation to the mechanical axis of the limb based on the kneejoint in an extended or near extended position.

Notably, neither of the aforementioned types of customised guides aredesigned so as to take into consideration the relationship of thefemoral and tibial resections in TKA/TKR and their orientation withrespect to each other in both extension and flexion. Accordingly,current patient-specific or customised resection members may fail toachieve a correctly aligned and/or balanced knee throughout its fullrange of motion. When a knee prosthesis is installed without beingproperly aligned and/or balanced multiple problems can arise. These caninclude stiffness, poor range of movement, pain, instability and,excessive shear forces at the interface between bony surfaces and theprosthesis, leading to subsequent failure of the prosthesis and surgicalrevision. Accordingly, an improved method of designing customisedresection guides of various design that achieve correct surgicalalignment of the knee joint together with optimal soft tissue balancingin both extension and flexion is required.

SUMMARY

The present invention is broadly directed to a method of designing apatient-specific or customised device for performing knee surgery and,in particular, TKR. The method may be performed to designpatient-specific or customised resection members and/or guides forprecise realignment and tensioning of the knee in both flexion andextension by way of coupling measurement of the femoral and tibial cutsin relation to the joint line and/or each other. The invention isfurther directed to patient-specific surgical devices produced therefromand their use in knee surgery.

In a first aspect, the invention is directed to a method of designing apatient-specific surgical device for performing knee surgery on thepatient including the steps of:

(i) determining a first alignment axis from one or a plurality of firstanatomical indicators on a patient-specific model of the patient's limbin extension

(ii) determining a second alignment axis from one or a plurality ofsecond anatomical indicators on a patient-specific model of thepatient's limb in flexion; and

(iii) designing the patient-specific surgical device based at leastpartly on the determined first and/or second alignment axes so that thepatient-specific surgical device is adapted to at least partly align afemur and/or a tibia of said limb with the first and/or second alignmentaxes.

In an embodiment, the method includes the step of using patient-specificanatomical data of a limb in both flexion and extension to create thepatient-specific model of said limb, prior to steps (i) and (ii).Preferably, the patient-specific model is or comprises a threedimensional model.

In particular embodiments, the one or plurality of first anatomicalindicators are selected from the group consisting of a central portionof a femoral head, a central portion of a femoral shaft, anintramedullary canal insertion point, a deepest portion of a trochleargroove, a central portion of an intercondylar notch, a central portionof a line extending between medial and lateral tibial spines, a centralportion of a talus, a central portion of a tibial shaft and an anteriorcruciate ligament tibial attachment point.

In one embodiment, the first alignment axis is or comprises a tibialmechanical axis, a femoral mechanical axis and/or a lower limbmechanical axis or an axis substantially parallel thereto.

In one embodiment, the method of the present aspect further includes thestep of rotating the tibia of the extended limb in a coronal planerelative to the femur, such that the tibial mechanical axis, the femoralmechanical axis and/or the lower limb mechanical axis are substantiallyparallel to the first alignment axis.

In one embodiment, the one or plurality of second anatomical indicatorsis or comprises a femoral anteroposterior axis, a tibial anteroposterioraxis and/or a tibial mechanical axis and the second alignment axis issubstantially parallel thereto. In another embodiment, the one orplurality of second anatomical indicators is or comprises atransepicondylar axis (TEA) and/or a posterior condylar axis and thesecond alignment axis is substantially perpendicular thereto.

Suitably, the method of the present aspect further comprises the step ofdetermining a flexion axis from one or a plurality of third anatomicalindicators on the patient-specific model of the limb, wherein the tibiais rotated relative to the femur about the flexion axis so as tosubstantially match the degree of flexion that exists between the femurand the tibia in the patient-specific anatomical data of the limb inextension.

In one embodiment, the one or plurality of third anatomical indicatorsare or comprise a lateral condyle arc centre and a medial condyle arccentre of the femur, such that the flexion axis extends therebetween.

In one embodiment, the one or plurality of second anatomical indicatorsis or comprises the flexion axis and the second alignment axis issubstantially perpendicular thereto.

Suitably, the method of the present aspect further comprises the step ofdetermining a joint line from one or a plurality of fourth anatomicalindicators. Preferably, the one or plurality of fourth anatomicalindicators are selected from the group consisting of a distal portion ofa medial condyle, a distal portion of a lateral condyle, a proximalportion of a medial tibial plateau, a proximal portion of a lateraltibial plateau, a central portion of a lateral meniscus and a centralportion of a medial meniscus.

Suitably, the method of the present aspect further comprises the step ofdetermining a distal resection plane of the femur when the knee is inextension from at least partly the joint line, the first alignment axisand/or a first dimension of a femoral prosthesis to be fitted on saidfemur. In one embodiment, the distal resection plane is substantiallyparallel to the joint line and/or is substantially perpendicular to thefirst alignment axis. In a further embodiment, the distance between thedistal resection plane and the joint line is substantially equal to thefirst dimension of the femoral prosthesis. In an alternative embodiment,the distance between the distal resection plane and the joint line isabout 0.5 mm to about 1.5 mm greater than the first dimension of thefemoral prosthesis.

Suitably, the method of the present aspect further comprises the step ofdetermining a proximal resection plane of the tibia when the knee is inextension from at least partly the distal femoral resection plane, thefirst dimension of the femoral prosthesis to be fitted on said femur, afirst dimension of a tibial prosthesis to be fitted on said tibia and/orthe joint line. In particular embodiments, the proximal resection planeis: (i) substantially parallel to the joint line of the knee inextension; (ii) substantially perpendicular to the first alignment axis;and/or (iii) substantially parallel to the distal femoral resectionplane of the knee in extension, when viewed in a coronal and/or sagittalplane of the limb. In alternative embodiments, the proximal resectionplane is at an angle of: (i) about 0.5 degrees to about 15 degreesrelative to the joint line and/or the distal femoral resection plane;and/or (ii) about 75 degrees to about 89.5 degrees relative to the firstalignment axis, when viewed in a sagittal plane of the limb. In oneembodiment, the distance between the proximal tibial resection plane andthe distal femoral resection plane are substantially equal to the sum ofthe first dimension of the femoral prosthesis and the first dimension ofthe tibial prosthesis. In another embodiment, the distance between theproximal resection plane and the distal resection plane is about 0.5 mmto about 2.5 mm greater than the sum of the first dimension of thefemoral prosthesis and the first dimension of the tibial prosthesis.

In one embodiment, the method of the present aspect further includes thestep of rotating the tibia of the flexed limb in a coronal planerelative to the femur, such that the proximal resection plane issubstantially perpendicular to the second alignment axis.

Suitably, the method of the present aspect further comprises the step ofdetermining a posterior resection plane of the femur when the knee is inflexion from at least partly the second alignment axis, the proximaltibial resection plane, the first dimension of the tibial prosthesis tobe fitted on the tibia and/or a second dimension of the femoralprosthesis to be fitted on the femur In one embodiment, the posteriorresection plane is substantially perpendicular to the second alignmentaxis and/or is substantially parallel to the proximal tibial resectionplane when viewed in a coronal and/or sagittal plane of the limb. In analternative embodiment, the proximal resection plane is at an angle of:(i) about 0.5 degrees to about 15 degrees relative to the posteriorresection plane; and/or (ii) about 75 degrees to about 89.5 degreesrelative to the second alignment axis, when viewed in a sagittal planeof the limb. In a further embodiment, the distance between the posteriorresection plane and the proximal resection plane is substantially equalto the sum of the first dimension of the tibial prosthesis and thesecond dimension of the femoral prosthesis. In an alternativeembodiment, the distance between the posterior resection plane and theproximal resection plane is about 0.5 mm to about 2.5 mm greater thanthe sum of the first dimension of the tibial prosthesis and the seconddimension of the femoral prosthesis.

In a particular preferred embodiment, the distal resection plane definesa distal femoral cut thickness and positioning, the proximal resectionplane defines a proximal tibial cut thickness and positioning and/or theposterior resection plane defines a posterior femoral cut thickness andpositioning such that a post-resection gap from said tibia to said femuris approximately equal in extension and in flexion of the knee.

Suitably, the method of any one of the preceding claims, wherein thepatient-specific model of the limb is created using both soft tissue andbony tissue data from the patient-specific anatomical data andoptionally further comprises the step of incorporating patient-specifickinematic and/or biomechanical data into the patient-specific model ofthe limb.

In certain embodiments, the method further comprises the step ofdetermining a position of one or a plurality of guide apertures in thepatient-specific surgical device for indicating or facilitatingpositioning of a resection member on the femur and/or tibia, wherein theresection member comprises one or a plurality of resection apertures forguiding a resection tool along the distal resection plane, the proximalresection plane, the posterior resection plane and/or an anteriorresection plane of the femur. Preferably, the guide apertures indicateor facilitate positioning of (i) a distal resection member on the femur;(ii) a proximal resection member on the tibia; and/or (iii) ananteroposterior resection member on the femur.

In other embodiments, the method further comprises the step ofdetermining a position of one or a plurality of resection apertures inthe patient-specific surgical device for guiding a resection tool alongthe distal resection plane, the proximal resection plane, the posteriorresection plane and/or an anterior resection plane of the femur.

In one embodiment, the patient-specific surgical device comprises aspacer for insertion between the femur and tibia to facilitate at leastpartial return of the knee joint to an appropriate alignment with thefirst alignment axis and/or the second alignment axis.

Suitably, the patient-specific surgical device is adapted to engage thefemur and/or the tibia in extension and/or flexion.

In certain embodiments, the patient-specific surgical device is designedso as to facilitate, at least partly, return of the knee to anappropriate and/or balanced soft tissue tension when in extension and/orflexion.

In particular embodiments, with respect to appropriate soft tissuetension, medial and/or lateral soft tissue laxity of the knee in flexionand/or extension is about 1° to about 7.0°.

In certain embodiments, with respect to balanced soft tissue tension,the difference between medial and lateral soft tissue laxity of the kneeis or less than about 5°.

In certain embodiments, the method further comprises the step ofobtaining the patient-specific anatomical data of the limb in flexionand/or extension.

In a second aspect, the invention provides a method of manufacturing apatient-specific surgical device comprising the steps of:

(i) designing the patient-specific surgical device at least partly bythe method of the first aspect; and

(ii) manufacturing the patient-specific surgical device based at leastpartly on the design of step (i).

In a third aspect, the invention provides a patient-specific surgicaldevice for performing knee surgery on said patient, wherein thepatient-specific surgical device is adapted to at least partly align afemur and/or a tibia of a limb with: (i) a first alignment axisdetermined from one or a plurality of first anatomical indicators on amodel of said patient's limb in extension; and/or (ii) a secondalignment axis determined from one or a plurality of second anatomicalindicators on a model of said patient's limb in flexion. Suitably, thepatient-specific surgical device is manufactured by the method of thesecond aspect.

In a fourth aspect, the invention provides a patient-specific surgicaldevice manufactured by the method of the second aspect.

In a fifth aspect, the invention provides a method of performing kneesurgery on a patient comprising the step of engaging thepatient-specific surgical device of the second aspect on a knee of saidpatient.

It will be appreciated that the indefinite articles “a” and “an” are notto be read as singular indefinite articles or as otherwise excludingmore than one or more than a single subject to which the indefinitearticle refers.

As used herein, unless the context requires otherwise, the words“comprise”, “comprises” and “comprising” will be understood to mean theinclusion of a stated integer or group of integers but not the exclusionof any other non-stated integer or group of integers.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the present invention may be readily understood and putinto practical effect, reference will now be made to the accompanyingillustrations, wherein like reference numerals are used to refer to likeelements.

FIG. 1: is a front view (A) and a medial side view (B) of a 3D model ofa left knee joint in extension and prior to alignment illustrating someexamples of first anatomical indicators and a first alignment axis.

FIG. 2: is a front view (A) and a medial side view (B) of the extendedknee joint of the 3D model of FIG. 1 illustrating an example of aligningthe tibia with the femur in the coronal plane.

FIG. 3: shows some examples of fourth anatomical indicators and theiruse in determining a joint line on a front view (A), a lateral side view(B) and a medial side view of the 3D model of FIG. 2.

FIG. 4: is a front view of the 3D model of FIG. 3 illustrating anexample of determining a distal femoral resection plane from a profileof a femoral prosthesis.

FIG. 5: is a front view (A) and medial side view (B) of the extendedknee joint of 3D model of FIG. 4 illustrating an example of determininga proximal tibial resection plane from profiles of a tibial prosthesisand/or a femoral prosthesis.

FIG. 6: is a front view (A) and medial side view (B) of the 3D model ofFIG. 5 illustrating an example of using a flexion axis to place the kneejoint of the 3D model in a flexed position.

FIG. 7: is a distal end view of the femur of the 3D model of FIG. 6illustrating some examples of second anatomical indicators and a secondalignment axis.

FIG. 8: is a front view of the flexed knee joint of the 3D model of FIG.7 prior to alignment (A) and a front view of this flexed knee jointillustrating an example of aligning the tibia with the femur in thecoronal plane (B).

FIG. 9: is a front view (A) and medial side view (B) of the flexed kneejoint of 3D model of FIG. 8 illustrating an example of determining aposterior femoral resection plane from the profiles of a tibialprosthesis and a femoral prosthesis.

FIG. 10: shows a front view of an embodiment of the device according toone aspect of the invention as applied to a left knee joint inextension.

FIG. 11: shows a side view of the device of FIG. 10 following fasteningto the femur and tibia.

FIG. 12: shows a front view of an embodiment of the device according toone aspect of the invention as applied to the left knee joint inflexion.

FIG. 13: shows a side view of the device of FIG. 12 following fasteningto the femur and tibia.

FIG. 14: shows a distal end perspective view (A) and a distal end viewof a femur having thereon an embodiment of a patient-specific surgicaldevice.

FIG. 15: shows a proximal end perspective view (A) and a proximal endview of a tibia having thereon an embodiment of a patient-specificsurgical device.

FIG. 16: Demonstration of the method of knee alignment moving fromdeformity to maximum co-axiality of the lower limb, tibial and femoralmechanical axes through a central portion of the knee (Medial andlateral laxity was measured in maximum extension and 20° flexion asdeviation from this point).

FIG. 17: Medial laxity compared to subjects grouped by maximum varusdeformity.

FIG. 18: Lateral laxity compared to subjects grouped by maximum varusdeformity.

FIG. 19: The relationship of the loaded and corrected HKA with referenceto neutral mechanical alignment +/−3°.

FIG. 20: Demonstration of correction of the limb axes in 90° flexion.The limb, femoral and tibial mechanical axes were placed in a co-axialor parallel position and then medial and lateral laxity was measured asdeviation from this point using the weight of the limb and manual force.

FIG. 21. Literature review of total coronal plane laxity arcs at 20° ofknee flexion compared to current study data. Coronal plane laxity (meanand standard deviation) of moderate to severe osteoarthritis versushealthy (non-arthritic) knees demonstrates no difference for groupstested under similar conditions.

FIG. 22. Patient satisfaction based on the validated satisfactionsection of the American knee society.

FIGS. 23 and 24. Demonstrate an embodiment (Scheme A) of a method ofproviding a patient-specific surgical device for a knee joint accordingto the invention described herein.

DETAILED DESCRIPTION

The present invention relates to a method of designing patient-specificsurgical devices for use during surgery and, in particular, TKR/TKA, fordetermining the thickness and positioning of one or more bony resectionsso as to achieve an appropriate alignment with one or more joint axesand/or an appropriate soft tissue balance of the knee joint in bothflexion and extension. While patient-specific or customised surgicaldevices are particularly suited for use in TKR/TKA, the presentinvention has general applicability to all types of joints (e.g.,elbows, shoulders, wrists and fingers) and replacement surgery thereofthat requires accurate gap balancing, joint alignment and/or soft tissuebalancing.

While the principles described herein are based on methods of providingpatient-specific surgical devices for humans, this invention may also beextended to other mammals such as livestock (e.g. cattle, sheep),performance animals (e.g. racehorses) and domestic pets (e.g. dogs,cats), although without limitation thereto.

The invention provides, in part, a method for determining resectionplanes to be used in designing a patient-specific or customised surgicaldevice. This patient-specific surgical device may generally be designedand created using patient-specific anatomical information or dataobtained from a knee joint in both flexion and extension. By way ofexample, the patient-specific surgical device may be a patient-specificor customised resection member for use in a total or partial jointreplacement surgical procedure. Non-limiting examples of customisedcutting guides include those by Zimmer, Signature guides by Biomet,Visionaire guides by Smith and Nephew, Trumatch guides by Depuy,Prophecy guides by Wright Medical, My Knee guides by Medacta and ShapeMatch guides by Otis Med. Alternatively, the patient-specific surgicaldevice may be a patient-specific or customised surgical or resectionguide, such as that disclosed in WO 2012/024306 and also describedherein.

Scheme A (FIGS. 23 and 24) demonstrates an embodiment of the method ofproviding a patient-specific surgical device for a knee joint accordingto the invention described herein.

Suitably, patient-specific anatomical data is obtained pre-operativelyusing one or more non-invasive imaging modalities, such as radiologicalimaging, computerised tomography (CT)/computerised axial tomography(CAT), magnetic resonance imaging (MRI), inclusive of full limb MM,ultrasound and/or other conventional means. The patient-specificanatomical data obtained therefrom may then be pre-processed and/orconverted to form a patient-specific model of the joint. Such apatient-specific model may include a two dimensional model (e.g.,radiographs, 2D slices of MRI) and/or a three dimensional model.Preferably, the patient-specific model is or comprises a threedimensional model, such as a three-dimensional (3D) computer aideddesign (CAD) model. Alternatively, the anatomical data may be used inits raw form to identify one or more anatomical indicators and/ordetermine an alignment and/or a flexion axis prior to being convertedinto a three dimensional model of the patient's limb. Generally,segmentation of the bone tissue, including osteophytes, from thepatient-specific anatomical data is performed to thereby create a threedimensional model of the affected knee joint.

In particular embodiments, the method of the present aspect includesobtaining a full scan of a patient's limb. For economical or efficiencypurposes, however, one or more partial scans of a patient's limb may beused rather than a full limb scan. In this regard, the exact location ofeach partial scan relative to a patient's limb may be carefully noted toensure that the scans are spaced apart in all directions correctlybefore determining one or more of the indicators and/or axes (e.g., thefirst alignment axis) provided herein. Furthermore, one or more partialscans may be utilised when a particular landmark, such as the truefemoral head centre or the transepicondylar axis, cannot be determinedand/or has been compromised due to trauma or gross deformation.

Suitably, the patient-specific anatomical data is used to determine afirst alignment axis from one or a plurality of first anatomicalindicators of a patient's limb prior to designing the customisedsurgical device for said patient. In this regard, a proximal portion anda distal portion of a patient's limb are typically identified. Thepatient's first alignment axis may then be determined by projecting andextending an imaginary line between the identified proximal and distalportions. Further, it would be appreciated that additional anatomicalindicators therebetween may be used in determining the first alignmentaxis of a patient's limb and the relationship between the overall axisand the intercalated segments thereof (e.g., the femoral and tibialmechanical axes).

By way of example, the proximal portion may be a centroid of the femoralhead and the distal portion may be the mid point of the superior mostapex of a talus bone. At the knee joint, the intercalated segments canbe marked by the sulcus of the intercondylar/trochlear groove or the midpoint of the line between the tibial spines. Accordingly, in oneembodiment, the first alignment axis is or comprises a patient'smechanical axis of the knee joint. To this end, the non-diseasedmechanical axis of the lower limb typically represents a line drawn fromthe centre of the femoral head to the centre of the ankle joint, passingthrough the knee just medial to the centre of the knee. In both thenormal and the osteoarthritic knee, this axis may be assessed as itscomponent femoral and tibial parts.

In an alternative embodiment, the first alignment axis is based at leastpartly on the pre-arthritic knee anatomy of the patient. A pre-arthriticmechanical or anatomical axis of the limb or knee joint may becalculated by manipulation of the patient-specific anatomical data withregards to the overall mechanical axis of the limb (i.e., lower limbmechanical axis) and the intercalated segments represented by thefemoral and tibial mechanical axes. This will typically place the limbin a ±3 degree range from neutral. Preferably, however, determining thefirst alignment axis in this manner will be individualised for thespecific patient's anatomy. For example, a patient whose knee joint mayrealign in 3 degrees of varus will generally not be placed in the “safe”zone at 2 degrees valgus as this represents a 5 degree deviation fromtheir “ideal” alignment.

Suitably, the first alignment axis of the limb is determined from one ora plurality of first anatomical indicators on the patient-specific modelof the limb in extension. In one embodiment, the first anatomicalindicators are selected from the group consisting of a central portionof a femoral head, a central portion of a femoral shaft, anintramedullary canal insertion point, a deepest portion of a trochleargroove, a central portion of an intercondylar notch, a central portionof a tibial spine, a central portion of a talus, a central portion of atibial shaft, an anterior cruciate ligament tibial attachment point.

Accordingly, in particular embodiments, the first alignment axis is amechanical axis of the limb, a femoral mechanical/anatomical axis and/ora tibial mechanical/anatomical axis. Furthermore, it would beappreciated that the first alignment axis may comprise more than onecomponent axis. By way of example, the first alignment axis may comprisea femoral mechanical axis and a tibial mechanical axis of the limb.Accordingly, in one embodiment, the first alignment axis is or comprisesa tibial mechanical/anatomical axis, a femoral mechanical/anatomicalaxis and/or a lower limb mechanical axis or an axis substantiallyparallel thereto.

In one embodiment, the central portion of the tibial spine orintercondylar eminence is determined, at least in part, by a lineextending between a lateral intercondylar tubercle and a medialintercondylar tubercle. Accordingly, the central portion of the tibialspine may be the midpoint of this line extending between the twointercondylar tubercles.

With regards to the femoral and/or tibial shafts, any central portion/sor point/s along the shaft, such as proximal, middle or distal centralportions, may be used as the first anatomical indicator by the skilledperson in performing the method of the present aspect.

It would be readily understood that the present invention should not belimited to the specific examples of anatomical indicators providedherein. Rather, the term anatomical indicator as used herein refers to areadily identifiable feature, specific area and/or landmark within or ona limb, such as an arm or leg, and/or a bone, such as a femur or tibia.In this regard, the anatomical indicators used for determining, forexample, a mechanical axis of the limb, a tibial mechanical/anatomicalaxis, a femoral mechanical/anatomical axis, a flexion axis, atransepicondylar axis (TEA), a posterior condylar axis, a tibialanteroposterior axis, a femoral anteroposterior axis and a joint line,are not to be limited to those anatomical indicators provided herein,but may also include other anatomical indicators as are known in theart.

In one embodiment, the method of the present aspect further includes thestep of rotating the tibia of the extended limb in a coronal planerelative to the femur, such that the tibial mechanical axis, the femoralmechanical axis and/or the lower limb mechanical axis are substantiallyparallel and/or co-axial to the first alignment axis. It would furtherbe appreciated that the method may further include the step oftranslating the tibia of the extended limb in a coronal plane relativeto the femur to facilitate alignment of the knee joint. Accordingly,following such rotation and/or translation of the tibia, two or more ofthe tibial mechanical axis, the femoral mechanical axis and the lowerlimb mechanical axis may be parallel and/or co-axial with each other. Anexample of such an alignment incorporating the tibial mechanical axis,the femoral mechanical axis and the lower limb mechanical axis isprovided in FIG. 16.

Suitably, the patient-specific anatomical data of the limb in extensionis obtained when the limb is in full extension (i.e., approximately 180degrees). The skilled artisan, however, would appreciate that this maynot be possible or feasible in all patients, owing, for example, to thepresence of pre-existing disease or deformities of the limb. By way ofexample, a patient with a flexion deformity or contracture of the kneemay be physically unable to fully straighten or extend the knee.Accordingly, in certain embodiments, the anatomical data of the limb inextension is obtained when the limb is not fully extended.

Preferably, for the patient-specific anatomical data of the limb inflexion, the knee joint is in approximately 85 to approximately 95degrees of flexion, including, but not limited to, 85, 85.5, 86, 86.5,87, 87.5, 88, 88.5, 89, 89.5, 90, 90.5, 91, 91.5, 92, 92.5, 93, 93.5,94, 94.5, 95 degrees or any range therein. In a particular preferredembodiment, the knee joint is in approximately 90 degrees of flexion.

It would be understood that for certain knee replacement systems, suchas the Journey™ range of knee replacement systems, assessment of thepatient-specific anatomical data of the limb in flexion may occur inknee joints in up to about 110 degrees of flexion. Accordingly, incertain embodiments, for the patient-specific anatomical data of thelimb in flexion, the knee joint is in approximately 95 to approximately110 degrees of flexion, including, but not limited to, 95, 95.5, 96,96.5, 97, 97.5, 98, 98.5, 99, 99.5, 100, 100.5, 101, 101.5, 102, 102.5,103, 103.5, 104, 104.5, 105, 105.5, 106, 106.5, 107, 107.5, 108, 108.5,109, 109.5, 110 degrees or any range therein. In a particular preferredembodiment, the knee joint is in approximately 105 degrees of flexion.

As used herein, the terms “approximately” and “about” refer totolerances or variances associated with numerical values recited herein.The extent of such tolerances and variances are well understood bypersons skilled in the art. Typically, such tolerances and variances donot compromise the structure, function and/or implementation of thedevices and methods described herein.

As demonstrated in Scheme A, the patient-specific anatomical data mayalso be used to determine a second alignment axis of a patient's limbprior to designing the customised surgical device for said patient. Inthis regard, the second alignment axis of the limb is suitablydetermined from one or a plurality of second anatomical indicators onthe patient-specific model of the limb in flexion. In one embodiment,the second anatomical indicator is or comprises a femoralanteroposterior axis, a tibial anteroposterior axis and/or a tibialanatomical/mechanical axis and the second alignment axis issubstantially parallel to either one or more of these axes. By way ofexample, the femoral anteroposterior axis may be the perpendicular tothe flexion and/or rotational axis of the femur.

It is to be understood that the second alignment axis may also be orcomprise any other femoral rotational axis known in the art, such as aposterior condylar axis; a transepicondylar axis (TEA); a rotationalaxis of the knee within the condyles and Whiteside's line. By way ofexample, the femoral anteroposterior axis may be determined, at least inpart from the TEA, Whitesides line, the posterior condylar axis or acombination of two or more of these axes. Accordingly, in oneembodiment, the second anatomical indicator is or comprises atransepicondylar axis (TEA) and/or a posterior condylar axis and thesecond alignment axis is substantially perpendicular thereto. In analternative embodiment, the second anatomical indicator is or comprisesa transepicondylar axis (TEA) and/or a posterior condylar axis and thesecond alignment axis is at an angle of about 80 degrees to about 100degrees, relative thereto. In particular embodiments of the presentinvention, the second alignment axis is at an angle of about 80, 81, 82,83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100degrees or any range therein, relative to the transepicondylar axisand/or the posterio condylar axis.

With respect to defining or determining the TEA, including the surgicaland/or anatomical TEA, this may be performed by any method known in theart. It is well established that the determination of the epicondylesmay lead to some confusion. On the lateral side of the femur, theprominence of the lateral condyle typically makes the lateral epicondyleeasy to define. However on the medial side, the most prominent part ofthe medial epicondyle, which is generally recognized on a CT scan as themost proximal ridge that gives insertion to the superficial collateralligament, may be used to determine the anatomical transepicondylar axis(aTEA). Alternatively, the depression below called the medialepicondylar sulcus may be used to define the surgical transepicondylaraxis (sTEA).

It would be appreciated that the patient-specific surgical deviceprovided by the method of the present aspect is suitably adapted to atleast partly align a femur and/or a tibia of said limb with the firstalignment axis, the second alignment axis and/or the flexion axis. Inthis regard, once the first and/or second alignment axes have beendetermined, any coronal realignment (if required) of the extended orflexed tibia relative to these axes can then be performed on thepatient-specific model of the patient's limb. Typically, the degree andtype of any deformity and/or defect, such as varus and valgus, of theknee joint in question are assessed on the patient-specific model. Thisthen allows for a determination of the amount of varus or valgusrotation (i.e. medial or lateral rotation of the tibia relative to itscorresponding femur in a coronal arc) that is required by thepatient-specific surgical device for angular correction of the tibia andthe knee joint line relative to the first and/or second alignment axes.

By way of example, if a patient were to exhibit a 10 degree varusdeformity of the knee (i.e., the distal part of the leg below the kneeis deviated inward 10 degrees from the long axis of the femur, resultingin a bowlegged appearance), then they would typically require acorrection of 7 to 13 degrees externally or valgus rotation of the tibiato achieve ideal joint alignment within the ±3 degree comfort zone. Thistranslation would proceed through the arc of approximately 7-13 degrees,but ceasing at the point limited by the soft tissues and the judgementof the surgeon or user regarding restoration of alignment. Once coronalalignment has been performed on the patient-specific model, weightbearing anteroposterior and/or lateral radiographs may be checked oroverlayed so as to check that the femoro-tibial alignment matches thatrequired by said radiographs.

Preferably, sagittal alignment and/or axial rotation of the tibia and/orfemur are unchanged from their native or pre-operative state, as per thepatient-specific anatomical data, after aligning the femur and/or tibiaof said limb with the first alignment axis and/or the second alignmentaxis. It would be appreciated by the skilled artisan, however, that inorder to achieve optimal coronal alignment additional translationaland/or rotational correction or alignment of the knee joint may berequired so as to account for, at least in part, any deformity causedby, for example, osteoarthritis and in doing so achieve a “true” neutralcoronal alignment of the limb.

As shown in Scheme A, application of the method suitably requires thedetermination of a joint line on the patient-specific model of thealigned knee in extension. As would be readily understood, engagement ofthe lateral and medial femoral condyles with the superior surface of thetibia of the extended knee establishes a joint line. Accordingly, suchdetermination may be made at least in part from one or a plurality offourth anatomical indicators, including, but not limited to, a distalportion of a medial condyle, a distal portion of a lateral condyle, aproximal portion of the medial tibial plateau, a proximal portion of thelateral tibial plateau, a central portion of a lateral meniscus and acentral portion of a medial meniscus. Accordingly, the joint line may bedetermined by drawing a line between the mid points of the bodies of thepatient's medial and lateral menisci in the coronal plane.Alternatively, it may be determined by drawing a line in relation to thedistal portion of the femoral condyle (i.e., medial and/or lateral)least altered or deformed by any osteoarthritic changes present therein.Suitably, the joint line is at an angle of about 85 degrees to about 95degrees to the first alignment axis of the femur. In particularembodiments, the joint line is at an angle of about 85, 86, 87, 88, 89,90, 91, 92, 93, 94, 95 degrees or any range therein, to the firstalignment axis of the femur. Preferably, the joint line is substantiallyperpendicular (i.e., about 90 degrees relative) to the first alignmentaxis of the femur.

A distal resection plane of the femur may then be established on thepatient-specific model from the determined joint line and a profile ordimension of a femoral component or prosthesis to be fitted on thefemur. By way of example, a profile or dimension may be selected fromone or more standard prosthetic devices, or custom prosthetic devices.The profile or dimension may be obtained from one or more product lineswhich may be from one or more implant manufacturers as are known in theart. Said profile or dimension typically indicate the size and/orpositioning of one or more bony resections needed to fit a particularstandard or custom prosthetic device. Preferably, the profile ordimension is of a prosthesis that has been sized and fittedappropriately for best coverage, bone conservation, flexion gapstability, patella tracking and/or placement without anterior femoralnotching.

Once selected by a user, the profile of a femoral prosthesis, and/or oneor more dimensions thereof, may be superimposed onto thepatient-specific model of the affected joint bone. Typically, the distalresection plane of the femur is determined from the profile and/or firstdimension of the femoral prosthesis and its positioning on the femurrelative to the joint line. Preferably, the femoral prosthesis ispositioned on the femur so as to allow for appropriate articulationthereof with an adjacent tibial prosthesis. Generally, the distalresection plane is also appropriately aligned on the patient-specificmodel relative to the first alignment axis.

Preferably, the distal resection plane is substantially parallel to thejoint line of the knee and/or is substantially perpendicular to thefirst alignment axis. Additionally, it would be understood that thedistal resection plane typically reproduces the limb alignmentdetermined for the knee joint in extension, such as in embodimentswherein the tibial mechanical, femoral mechanical and lower limbmechanical axes are substantially parallel and/or coaxial.

In one embodiment, the distance between the joint line and the distalresection plane is substantially equal to the first dimension of thefemoral prosthesis to be fitted on the femur. In alternativeembodiments, the distance between the joint line and the distalresection plane is about 0.5 mm to about 1.5 mm greater than the firstdimension of the femoral prosthesis to be fitted on the femur.Typically, this distance of about 0.5 mm to about 1.5 mm would dependupon surgeon preference, but its application is based on previous workdemonstrating this “over resection” may be necessary in some patients tonormalise periarticular tissue tension. Accordingly, this additionaldistance of about 0.5 mm to about 1.5 mm may allow for the maintenanceof normal soft tissue tension post surgery in the majority of cases. Thedetermination of this “over resection” may also be in part determined byinputs from patient-specific kinematic and/or biomechanical modelsaccounting for bone and soft tissue inputs.

This additional distance may also be applied unilaterally to thepatient's knee joint and in particular to the non-diseased side of thejoint. By way of example, in a varus knee, the distance between alateral portion of the joint line and a lateral portion of the distalresection plane may be about 0.5 mm to about 1.5 mm greater than thefirst dimension of the femoral prosthesis to be fitted on the femur.Conversely, in a valgus knee, the distance between a medial portion ofthe joint line and a medial portion of the distal resection plane may beabout 0.5 mm to about 1.5 mm greater than the first dimension of thefemoral prosthesis to be fitted on the femur.

Suitably, CAD programs, biomechanical modelling software and FiniteElement Analysis software or the like may be utilised to virtually testa given prosthesis' performance. For example, once a profile is fixed inspace on a model of a patient's limb, a prosthesis having the sameprofile as a bony interface may be superimposed on the patient-specificmodel. Software may perform iterative test runs to predict whether ornot small adjustments to the positioning of the prosthesis are necessaryto optimise performance of both the kinematics of the prosthesis/kneeand tension/balance of the periarticular soft tissues.

A proximal resection plane of the tibia may also be established from thepatient-specific model of the patient's knee. In particular embodiments,wherein the knee joint is in extension, the proximal resection plane ofthe tibia is determined from at least partly the distal resection planeof the femur, the first dimension of the femoral prosthesis to be fittedon said femur, the joint line and/or a first dimension of a tibialprosthesis to be fitted on said tibia. To this end, and similar to thatdescribed above for the distal resection plane, the profiles, or one ormore dimensions thereof, of the femoral and tibial prostheses may besuperimposed onto the patient-specific model of the affected knee joint.Typically, the proximal resection plane of the tibia is determined fromthe distal resection plane and appropriate positioning of the femoraland tibial prostheses on the femur and tibia respectively of thepatient-specific model that allows for suitable articulationtherebetween. Generally, the proximal resection plane is alsoappropriately aligned on the patient-specific model relative to thefirst alignment axis.

Typically, tibial prostheses include a tibial bearing or meniscalreplacement component having a concave articular portion configured forarticulation with the femoral prosthesis and a tibial tray to which thebearing or meniscal replacement component of the tibial prosthesis maybe secured. The tibial tray is generally secured to the bone stock of aresected proximal tibia. As is well known in the art, the bearing ormeniscal replacement component is used to provide an appropriate levelof friction and contact area at the interface between the femoralcomponent and the tibial bearing component.

In one embodiment, the distance between the distal resection plane andthe proximal resection plane is substantially equal to the sum of thefirst dimension of the femoral prosthesis and the first dimension of thetibial prosthesis. In alternative embodiments, the distance between thedistal resection plane and the proximal resection plane is about 0.5 mmto about 2.5 mm greater than the sum of the first dimension of thefemoral prosthesis and the first dimension of the tibial prosthesis. Ashereinbefore described, this additional distance may be appliedunilaterally to the patient's knee joint, such as to the non-diseasedside of the joint.

In other embodiments, wherein the knee joint is in extension, theproximal resection plane of the tibia is determined from at least partlythe joint line and/or the first dimension of the tibial prosthesis to befitted on the tibia. To this end, the proximal resection plane of thetibia may be determined from the profile and/or first dimension of thetibial prosthesis and its positioning on the tibia relative to the jointline. Preferably, the tibial prosthesis is positioned on the tibia inthe patient-specific model so as to allow for appropriate articulationthereof with an adjacent femoral prosthesis.

In one embodiment, the distance between the proximal resection plane andthe joint line is substantially equal to the first dimension of thetibial prosthesis. In alternative embodiments, the distance between thejoint line and the proximal resection plane is about 0.5 mm to about 1.5mm greater than the first dimension of the tibial prosthesis.

In particular embodiments, when the patient-specific model of the kneeis in extension, the proximal resection plane is: (i) substantiallyparallel to the joint line; (ii) substantially parallel to the distalfemoral resection plane; and/or (iii) substantially perpendicular to thefirst alignment axis when viewed in relation to the coronal and/orsagittal plane of the limb of the relevant knee joint.

It would be appreciated by the skilled person, however, that theproximal resection plane may possess an anteroposterior slope whenviewed in the sagittal plane. Accordingly, the proximal resection planemay only be substantially parallel to the joint line, substantiallyparallel to the distal femoral resection plane; and/or substantiallyperpendicular to the first alignment axis when viewed in a coronalplane, but not the sagittal plane owing to the aforementionedanteroposterior slope. This slope is typically appropriate to theprosthesis to be fitted on the tibia and the individual patient'sanatomy. Generally, this slope of the proximal resection plane isbetween about 0 and about 15 degrees, including, but not limited to,0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0,7.5, 8.0, 8.5, 9.0, 9.5, 10.0, 10.5, 11.0, 11.5, 12.0, 12.5, 13.0, 13.5,14.0, 14.5, 15.0 degrees or any range therein.

Accordingly, in particular embodiments the proximal resection plane isat an angle of (i) about 0.5 degrees to about 15 degrees relative to thejoint line and/or the distal femoral resection plane; and/or (ii) about75 degrees to about 89.5 degrees relative to the first alignment axis,when viewed in a sagittal plane of the limb.

Suitably, the method of the present aspect further comprises the step ofdetermining a flexion axis which preferably extends through the condylararc centres of the femur so as to at least partly facilitatereproduction of the relationship between the patient's femur and tibiain the sagittal and coronal planes. This position may also be partlydetermined by patient specific kinematic and/or biomechanical data andmodels, as described herein.

As noted previously, it may not be possible to fully extend or nearfully extend a patient's knee, such as in cases of a fixed flexiondeformity. Accordingly, when designing a patient-specific surgicaldevice, the tibia may need to be rotated relative to the femur about theflexion axis on the patient-specific model so as to substantially matchthe degree of flexion that exists between the femur and tibia in thepatient's “fully” extended knee preoperatively. Additionally, the tibiamay need to be translated relative to the femur on the patient-specificmodel so as to substantially match the spatial relationship that existsbetween the femur and tibia preoperatively. This rotation and/ortranslation of the tibia femur on the patient-specific model may beperformed prior to determining the proximal and/or distal resectionplane. Alternatively, the proximal and/or distal resection plane may bedetermined and then similarly rotated relative to the femur about theflexion axis on the patient-specific model so as to match the degree offlexion of the native knee. As such, in particular embodiments, theproximal resection plane and/or the distal resection plane are at leastpartly determined from the flexion axis.

Once the proximal and distal resection planes have been determined fromthe patient-specific model of the knee in extension or near extension,the posterior femoral resection plane may be determined from thepatient-specific model of the knee in flexion. To this end, the tibia ofthe patient-specific model may be rotated relative to the femur aroundthe flexion axis, such that a tibial component of the mechanical axis isapproximately at 90 degrees to a femoral component of the mechanicalaxis in the sagittal plane. The position of the 3D model of the tibiarelative to the femur in 90 degrees of flexion can be verified frompatient-specific anatomical data obtained from the patient's knee imagedpre-operatively at approximately 90 degrees of flexion.

Once the patient-specific model of the knee has been rotated to a flexedposition, a second alignment axis of the limb may then be determinedfrom one or a plurality of second anatomical indicators thereon. It isto be understood that the second alignment axis may be or comprise anyfemoral rotational axis known in the art. In one embodiment, the secondanatomical indicator is or comprises a femoral anteroposterior axisand/or a tibial anteroposterior axis and the second alignment axis issubstantially parallel thereto. In a further embodiment, the secondanatomical indicator is or comprises a TEA and/or a posterior condylaraxis and the second alignment axis is substantially perpendicularthereto.

Once the second alignment axis has been determined, coronal alignment ofthe 90 degree rotated tibia relative to the second alignment axis may beperformed. Accordingly, in one embodiment, the method of the presentaspect further includes the step of rotating the tibia of the flexedlimb in a coronal plane relative to the femur, such that the proximalresection plane is substantially perpendicular to the second alignmentaxis. Suitably, after performing coronal alignment of the tibia, theproximal resection plane is at an angle of about 85 degrees to about 95degrees (i.e., about 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95 degreesor any range therein) to the second alignment axis when viewed in thecoronal and/or sagittal planes of the knee joint. Preferably, afterperforming coronal alignment of the tibia, the proximal resection planeis substantially perpendicular to the second alignment axis when viewedin the coronal and/or sagittal planes of the knee joint. As notedearlier, however, the proximal resection plane may alternatively possessan anteroposterior slope (e.g., between about 75 and about 90 degrees,or any range therein, relative to the second alignment axis) when viewedin the sagittal plane.

Suitably, the posterior resection plane of the femur when the knee is inflexion can now be determined at least partly from the proximal tibialresection plane, the first dimension of the tibial prosthesis to befitted on the tibia and/or a second dimension of the femoral prosthesisto be fitted on the femur. In one embodiment, the distance between theposterior femoral resection plane and the proximal tibial resectionplane is substantially equal to the sum of the first dimension of thetibial prosthesis and the second dimension of the femoral prosthesis. Inan alternative embodiment, the distance between the posterior resectionplane and the proximal resection plane is about 0.5 mm to about 2.5 mmgreater than the sum of the first dimension of the tibial prosthesis andthe second dimension of the femoral prosthesis.

In certain preferred embodiments, wherein the knee joint is in flexion,the proximal tibial and/or posterior femoral resection planes aresubstantially perpendicular to the second alignment axis and/or ananteroposterior axis of the femur. As hereinbefore described, theproximal resection plane may be at an anteroposterior slope when viewedin the sagittal plane. Accordingly, in the flexed knee, the proximalresection plane may only be substantially perpendicular to the secondalignment axis and/or an anteroposterior axis of the femur and/orsubstantially parallel to the posterior femoral resection plane whenviewed in the coronal plane of the limb, but not the sagittal planeowing to the aforementioned anteroposterior slope.

Accordingly, in particular embodiments, the proximal resection plane is(i) at an angle of about 0.5 degrees to about 15 degrees relative to theposterior resection plane; and/or (ii) at an angle of about 75 degreesto about 89.5 degrees relative to the second alignment axis and/or theanteroposterior axis of the femur, when viewed in a sagittal plane ofthe limb.

Preferably, the second alignment axis ensures surface contact betweenthe least affected posterior femoral condyle and an opposing portion ofthe tibial plateau after coronal alignment of the flexed knee joint.

With respect to the angle of the knee in flexion, the skilled addresseewould recognize that this angle will typically be dependent upon thesubsequent TKR system to be used by the surgeon. In this regard, mostTKR systems require the flexion angle to be approximately 85 to 95degrees, and in particular about 90 degrees. There are, however, TKRsystems such as the Journey TKR systems by Smith and Nephew that requirethe flexion angle to be approximately 100 to 110 degrees. Nonetheless,it should be readily apparent to the skilled artisan that the devices ofthe first and second aspect are not to be limited to any particularcustomised TKR system or method.

Suitably, the distal resection plane defines a distal femoral cutthickness and positioning, the proximal resection plane defines aproximal tibial cut thickness and positioning and/or the posteriorresection plane defines a posterior femoral cut thickness andpositioning such that a post-resection gap from the tibia to the femuris approximately equal in extension and flexion of the knee joint andthat by using this method a ligament tension more individualised to thatspecific patient will be achieved. One skilled in the art wouldappreciate that a patient's flexion gap may be approximately equal tothat of their extension gap, however, a flexion gap may be more lax thanits corresponding extension gap and there is often some mediolateralasymmetry in both flexion and extension in the native knee. Current TKRmethods may not produce balanced flexion and extension gaps or mayattempt to “normalise” the gaps which may act to impose incorrect tissuetension on an individual knee. Hence the need to provide an improvedmethod of planning resection planes for patient-specific surgicaldevices for TKR which will produce balance and individualisation of thisbalance to specific patients.

In one embodiment, the distal femoral resection plane is substantiallyparallel to the proximal tibial resection plane when the knee joint isin extension, such that an extension gap from the tibia to the femur issubstantially rectangular after performing distal and proximalresections. In a further embodiment, the proximal tibial resection planeis substantially parallel to the posterior femoral resection plane whenthe knee joint is in flexion, such that a flexion gap from the tibia tothe femur is substantially rectangular after performing proximal andposterior resections.

It would be appreciated by the skilled artisan, that one or more of theresection planes described herein may be determined, at least in part,with the aid of inputted soft tissue data, as shown in Scheme A. In thisregard, segmented soft tissue and bone data from the patient-specificanatomical data may be utilized, such as in a multi-body inversedynamics system, to produce a model of the kinematics and/orbiomechanics of the patient's knee joint. Such models may allow foradjustment or fine tuning of, for example, the coronal alignment of therelevant tibia and femur, and in particular the position of the tibiarelative to the femur in the flexed knee, based on the their specifickinematics. Accordingly, this adjustment or fine tuning may influencethe determination of the distal, proximal and/or posterior resectionplanes in said patient.

FIGS. 1 to 9 illustrate one embodiment for a method of designing apatient-specific surgical device of the present invention. FIG. 1demonstrates a front view and a medial side view of an embodiment of a3D model of a knee joint 700 of a left leg in extension prior to coronalalignment of the tibia 900 and the femur 800 by way of the firstalignment axis (x). In the embodiment shown, the first alignment axis(x) is determined by projecting and extending an imaginary vertical linefrom a central portion of the femoral head (not shown) through a deepestportion of a trochlear groove 801 of the femur 800 (i.e., a femoralmechanical or anatomical axis). A tibial mechanical or anatomical axis901 is also determined on the 3D model by extending a line from acentral portion of the tibial spine 902 to a central portion of thetalus (not shown).

In FIG. 2, the tibia 900 has now been rotated laterally in the coronalplane of the knee joint 700 relative to the femur 800, such that thetibial mechanical axis 901 is substantially parallel or co-axial to thefirst alignment axis (x) in the coronal plane. The spatial position ofthe tibial mechanical axis 901 prior to this lateral rotation (denotedas 901 a) and the angle of coronal correction (p) are also provided inFIG. 2a . As can be observed from FIGS. 1b and 2b , this coronalalignment of the knee joint 700 in extension is performed with little orno change to the alignment and axial rotation of the tibia 900 relativeto the femur 800 when viewed in the sagittal plane.

Although not shown in FIG. 2, alignment of the extended knee joint 700in the coronal plane may also be performed at least in part by use ofthe lower limb mechanical axis (not shown) in addition to the tibial andfemoral mechanical axes 901 (x). It would be understood, that the lowerlimb mechanical axis is typically that which extends between the femoralhead center and the talus center, as hereinbefore described. To thisend, the tibia 900 may be rotated laterally and/or translated in thecoronal plane of the knee joint 700 relative to the femur 800, such thatthe lower limb mechanical axis is substantially parallel or co-axial tothe tibial mechanical axis 901 and/or the femoral mechanical axis (x) inthe coronal plane. In this regard, any of the lower limb mechanicalaxis, the tibial mechanical axis and the femoral mechanical axis mayserve as the first alignment axis (x). An embodiment of this method ofalignment is provided in FIG. 16.

From FIG. 3, once the femur 800 and the tibia 900 are aligned in thecoronal plane a joint line 5 is determined and drawn on the 3D model ofthe knee joint 700 from one or more anatomical indicators. In theembodiment provided, the joint line 5 is drawn as a line or plane on the3D model extending between and through a distal portion of the medialcondyle 802 and a distal portion of a lateral condyle 803 of the femur800. Further, the joint line 5 can be observed to be substantiallyperpendicular to the first alignment axis (x) in both the coronal andsagittal planes of the knee joint 700. The joint line 5 is also shown tobe substantially perpendicular to the tibial mechanical axis 901 in thecoronal plane of the aligned knee joint 700 in extension.

As shown in the embodiment of FIG. 4, a profile or a dimension of afemoral prosthesis 10 to be fitted on the femur 800 is then superimposedon the 3D model of the knee joint 700 in extension. A distal resectionplane (a) of the femur 800 is then determined from the 3D model as thedistance from the joint line 5 to the distal resection plane (a) equalsa first dimension 11 of the femoral prosthesis 10. In the embodimentprovided, the first dimension 11 of the femoral prosthesis 10 is thedistance between the distal articulation surface of the femoralprosthesis 10 and the distal interface of the femoral prosthesis 10 withthe femur 800. In a coronal plane, as can be observed from FIG. 4, thedistal resection plane (a) is perpendicular to the first alignment axis(x) and the tibial mechanical axis 901 and substantially parallel to thejoint line 5 of the knee joint 700 in extension. Although not shown inFIG. 4, the distal resection plane is also perpendicular to the firstalignment axis (x) and substantially parallel to the joint line 5 in thesagittal plane.

In FIG. 5, a profile or a dimension of a tibial prosthesis 20 to befitted on the tibia 900 is then superimposed on the 3D model of the kneejoint 700 in extension so as to facilitate determining a proximalresection plane (b) of the tibia 900. In this regard, the proximalresection plane (b) is calculated as that distance from the joint line 5equal to the first dimension 21 of the tibial prosthesis 20 to be fittedon the tibia 900. As can be seen in FIG. 5b , the tibial prosthesis 10when suitably fitted on the tibia 900 resides in an anteroposteriorslope relative to the joint line 5 and the distal resection plane (a)when viewed in the sagittal plane. Accordingly, the proximal resectionplane of the embodiment in FIG. 5 is also determined to be at an angle(z) to the joint line 5 and at an angle (v) to the first alignment axis(x) when viewed in the sagittal plane. The angle (z) is typicallybetween 0 and 15 degrees. Accordingly, the angle (v) is typicallybetween 75 and 90 degrees

As illustrated in FIG. 6 and following determination of the distalfemoral resection plane (a) and the proximal tibial resection plane (b),the tibia 900 is rotated relative to the femur 800 about a flexion axis50 so as to provide a 3D model of the knee joint 700 in flexion. Priorto this rotation, the flexion axis 50 is determined on the 3D model ofthe knee joint 700 by extending a line between a medial condyle arccentre 807 and a lateral condyle arc centre 808. In the embodimentprovided, this rotation results in the tibial mechanical axis 901 beingsubstantially perpendicular to the first alignment axis (x) or a femoralmechanical or anatomical axis (not shown) (i.e., angle (q) isapproximately 90°).

FIG. 7 demonstrates that following placement of the 3D model of the kneejoint 700 in a flexed position a second alignment axis (y) is determinedfrom one or a plurality of second anatomical indicators. In theembodiment provided, the transepicondylar axis (TEA) 816 has been drawnon the model, as a line extending between the medial and lateralepicondyles 811, 812 of the femur 800. In addition, a posterior condylaraxis 817 extending between posterior portions of the lateral and medialcondyles 813, 814 of the femur 800 has been drawn on the 3D model. Thesecond alignment axis (y) can then be determined on the 3D model of theknee joint 700 in flexion from these second anatomical indicators.

In the embodiment provided, the second alignment axis (y) issubstantially parallel to the anteroposterior axis 815, is substantiallyperpendicular to the TEA 816 and is at an angle (r) to the posteriorcondylar axis 817. As would be appreciated, only one of theanteroposterior axis 815, the TEA 816 and the posterior condylar axis817 may be determined so as to facilitate calculating the secondalignment axis (y). Two or more of these axes, however, may be used toimprove a user's accuracy in calculating the second alignment axis (y)on the 3D model of the knee joint 700 in flexion.

In FIG. 8 and following determination of the second alignment axis (y),coronal alignment of the tibia 900 relative to the femur 800 has beenperformed on the 3D model of the knee joint 700 in flexion. As can beobserved in FIG. 8a , the tibial mechanical axis 901 is at an angle (f)relative to the second alignment axis (y) prior to coronal alignment ofthe knee joint 700 in flexion. In FIG. 8b , the tibia 900 has now beenrotated medially by an amount equal to the angle (f) in a coronal plane,such that the tibial mechanical axis 901 is now substantially parallelto the second alignment axis (y) on the 3D model. Further, in FIG. 8bthe proximal resection plane (b) is now substantially perpendicular tothe second alignment axis (y) when viewed in a coronal plane.

As shown in FIG. 9, a profile of the femoral prosthesis 10 and a profileof the tibial prosthesis 20 are then superimposed on the 3D model of theknee joint 700 in flexion so as to facilitate determining a posteriorresection plane (c). To this end, FIG. 9 illustrates that the posteriorresection plane (c) has been calculated as that distance from theproximal resection plane (b) equal to the sum of a second dimension 12of the femoral prosthesis 10 and the first dimension 21 of the tibialprosthesis 20. In the embodiment provided, the second dimension 12 ofthe femoral prosthesis 10 is the distance between the posteriorarticulation surface of the femoral prosthesis 10 and the posteriorinterface of the femoral prosthesis 10 with the femur 800.

As can be seen in FIG. 9b , when suitably fitted on the tibia 900 of theflexed knee joint 700, the tibial prosthesis 10 resides in ananteroposterior slope relative to the posterior resection plane (c) andthe second alignment axis (y) (not shown) when viewed in the sagittalplane. Accordingly, the posterior resection plane of the embodiment inFIG. 9 is also determined to be at an angle (l) relative to the proximalresection line (b) and at an angle (m) relative to the tibial mechanicalaxis 901 and the second alignment axis (y) (not shown) when viewed inthe sagittal plane. The angle (l) is typically between 0 and 15 degrees.Accordingly, the angle (m) is typically between 75 and 90 degrees.

Once the patient-specific distal, proximal and/or posterior resectionplanes have been determined, a patient-specific or customised surgicaldevice may be created and provided.

The patient-specific surgical device preferably conforms in some way toa patient's bone and is generally specific to said patient. For example,the bone-contacting or -engaging surfaces of the patient-specificsurgical device may comprise one or more portions or surfaces havingmirrored contours of a patient's bone thereon, or the bone-contacting or-engaging surfaces may comprise a few suitably located contact pointsthat reversibly lock the patient-specific surgical device on thepatient's bone. In addition, the patient-specific surgical device maycomprise one or more means, such as a fixation aperture adapted toreceive a fastening member therethrough, so as to facilitate reversiblefixation of the device to the patient's bone.

In particular embodiments, the patient-specific surgical devicecomprises one or a plurality of means, such as resection apertures orslots, for guiding a resection or cutting tool along the distalresection plane, the proximal resection plane and/or the posteriorresection plane. Accordingly, the patient-specific surgical devicesthemselves may function as patient-specific distal femoral, proximaltibial and/or anteroposterior femoral resection members with theresection apertures or slots positioned therein via, at least in part,the methods of designing a patient-specific surgical device describedherein rather than previous positioning algorithms.

In other embodiments, the patient-specific surgical device comprise oneor a plurality of guide apertures, which indicate or correlate todesired surgical alignments by, for example, inserting alignment pins.In this regard, the alignment pins are inserted along the trajectoriesdefined by the guide apertures, after which the patient-specificsurgical device is removed from the knee leaving the alignment pins inplace. Alternatively, holes can be drilled in a patient's bone throughthe guide apertures and alignment pins, posts, or other components canbe aligned relative to the holes after the patient-specific surgicaldevice has been removed. The alignment pins can subsequently orient aresection member, for example, by sliding said resection member thereon.When suitably attached to the alignment pins, the resection memberpreferably comprises one or a plurality of resection apertures forguiding a resection or cutting tool along the distal resection plane,the proximal resection plane and/or the posterior resection plane.

As would be understood by the skilled artisan, in embodiments of thepresent invention wherein one or a plurality of guide aperturesfacilitate the positioning of a resection member, the position andorientation (e.g., angle relative to the bone) of these guide apertures,and hence alignment pins received therethrough, may be influenced ordetermined by the particular type of resection member to be used by thesurgeon.

In order to facilitate the positioning of a resection member, theposition and orientation of these guide apertures may first facilitatethe positioning of an alignment device on the knee joint, such as thatdescribed in WO2012/024306. In this regard, the alignment device mayengage the tibia and/or femur directly or indirectly, such as by way ofa resection member, so as to provide surgical alignment of the bonyresections required relative to those of the tibia and/or femur whilethe knee joint is in extension or flexion. For example, the alignmentdevice can be shaped so as to engage the cutting slot of a distalfemoral resection member. Once engaged with said distal femoralresection member, the alignment device may include slots or aperturesfor aligning the proximal resection of the tibia with the distalresection of the femur in a coronal plane whilst the knee joint is inextension. The alignment device may further define guide apertures orslots through which alignment pins for the placement of a resectionmember thereon may be inserted.

Preferably, the patient-specific surgical device provided hereincomprises a spacer for insertion between the femur and tibia to at leastpartly facilitate return of the knee joint to an appropriate alignmentwith the first and/or second alignment axis and/or an appropriate softtissue balance. The spacer of the device is typically customized and/orpre-determined according to the patient-specific anatomical data. Inthis regard, a spacer may extend to either the lateral or medial sidesof the knee joint or both. This would depend upon the individual'spreoperative knee anatomy as determined by, for example, magneticresonance imaging (MRI) or computed tomography (CT), and the presence ofany anatomical deformities and/or defects, such as varus or valgus. Byway of example, the skilled artisan would readily recognise that a kneedemonstrating varus deformity may require a spacer placed only mediallyin the joint to achieve a desired joint alignment and/or soft tissuebalancing. As such, the position, size and shape of the spacer are notto be limited to any particular knee defect and/or deformity.

In particular embodiments, the spacer may extend into the joint spacebetween the femur and the tibia, for example, substantially to thecenter of the joint space along an anterior-posterior direction. Thespacer may extend a greater or lesser distance into the joint space, andmay extend completely through the joint space. In certain embodiments,the shape of the spacer preferably allows for the presence of unresectedanterior and/or posterior cruciate ligaments within the knee joint.

The spacer may optionally include and upper surface that ispatient-matched to conform to a distal surface of the femur, such as adistal condylar surface. Additionally, the spacer may include a lowersurface that is patient-matched to conform to a proximal surface of thetibia, such as the proximal tibial plateau. Engagement of one or more ofthe patient-matched surfaces to the knee can contribute furtherstability, such as rotational and/or anteroposterior stability, to theengagement of the patient-specific surgical device to the knee.

It would also be appreciated that the spacer to be designed on the 3Dmodel of the knee joint may be only virtual in nature and not constitutepart of the final design of the patient specific surgical device.Accordingly, separate patient specific surgical devices for the femurand/or tibia may be manufactured by the design method described hereinthat do not include a spacer (see, for example, the embodiments ofpatient specific surgical devices in FIGS. 14 and 15). Such patientspecific surgical devices, however, would allow for appropriate softtissue balancing and knee joint alignment despite the absence of thespacer, as the position of the required resection plane/s have typicallybeen determined after such balancing and alignment on the 3D model ofthe patient's knee joint. Further, a spacer may not be requiredregardless of whether the patient specific surgical device has beendesigned for directly resecting the femur and/or tibia or facilitatingpositioning of a resection member on the femur and/or tibia.

In certain embodiments, the patient-specific surgical device is designedso as to facilitate, at least partly, return of the knee to anappropriate and/or balanced soft tissue tension when in extension and/orflexion. Balanced soft tissue tension requires placing the soft tissuessurrounding and/or interconnecting the bones of the knee, such as thatof the medial and lateral knee, at an approximately equal or similartension relative to one another when the femur and its correspondingtibia are placed in a desired alignment as determined by the surgeon.Preferably, an appropriate soft tissue tension is approximately equal orsimilar to the physiological tension of these soft tissues in the nativeknee at rest. In this regard, a patient's soft tissue tension may becompared to that of a control of reference sample or population.Non-limiting examples of the soft tissues surrounding and/orinterconnecting the bones of the knee include the medial and lateralcollateral ligaments, the anterior and posterior cruciate ligaments, theposteromedial and posterolateral ligamentous structures and theposterior capsule. It would be appreciated that the present invention isnot to be limited to any particular means of measuring soft tissuetension of the knee, which may include, for example, an arthrometer, adynamometer, radiography, MRI, computer assisted surgery and a kneejoint tension meter.

For the purposes of the present invention, such appropriate and/orbalanced soft tissue tensioning may be achieved during and/or aftersurgery. Further, an appropriate and/or balanced soft tissue tension maybe transient in nature.

In particular embodiments, with respect to an appropriate soft tissuetension, medial and/or lateral soft tissue laxity of the knee in flexionand/or extension is about 1° to about 7.0°. In particular embodiments ofthe present invention, the medial and/or lateral soft tissue laxity ofthe knee in flexion and/or extension is about 1.0°, 1.1°, 1.2°, 1.3°,1.4°, 1.5°, 1.6°, 1.7°, 1.8°, 1.9°, 2.0°, 2.1°, 2.2°, 2.3°, 2.4°, 2.5°,2.6°, 2.7°, 2.8°, 2.9°, 3.0°, 3.1°, 3.2°, 3.3°, 3.4°, 3.5°, 3.6°, 3.7°,3.8°, 3.9°, 4.0°, 4.1°, 4.2°, 4.3°, 4.4°, 4.5°, 4.6°, 4.7°, 4.8°, 4.9°,5.0°, 5.1°, 5.2°, 5.3°, 5.4°, 5.5°, 5.6°, 5.7°, 5.8°, 5.9°, 6.0°, 6.1°,6.2°, 6.3°, 6.4°, 6.5°, 6.6°, 6.7°, 6.8°, 6.9°, 7.0°, or any rangetherein. As would be appreciated by the skilled artisan, thismeasurement of medial and/or lateral soft tissue laxity may varyaccording to the means used to measure such laxity.

In certain embodiments, with respect to a balanced soft tissue tension,the difference between medial and lateral soft tissue laxity of the kneein flexion and/or extension is equal to or less than about 5°. Inparticular embodiments of the present invention, the difference betweenmedial and lateral soft tissue laxity of the knee is about 0°, 0.1°,0.2°, 0.3°, 0.4°, 0.5°, 0.6°, 0.7°, 0.8°, 0.9°, 1.0°, 1.1°, 1.2°, 1.3°,1.4°, 1.5°, 1.6°, 1.7°, 1.8°, 1.9°, 2.0°, 2.1°, 2.2°, 2.3°, 2.4°, 2.5°,2.6°, 2.7°, 2.8°, 2.9°, 3.0°, 3.1°, 3.2°, 3.3°, 3.4°, 3.5°, 3.6°, 3.7°,3.8°, 3.9°, 4.0°, 4.1°, 4.2°, 4.3°, 4.4°, 4.5°, 4.6°, 4.7°, 4.8°, 4.9°,5.0°, or any range therein. Such soft tissue laxity of the knee may bemeasured by any means known in the art, such as those hereinbeforedescribed.

FIGS. 10 and 11 show one embodiment of a patient-specific surgicaldevice 100 designed by the method hereinbefore described to be used whenthe knee joint 700 is in extension. The patient specific device 100comprises an engaging portion 110 adapted to engage one or more anteriorsurfaces 810 of a distal portion 805 of the femur 800 and one or moreanterior surfaces 910 of a proximal portion 905 of the tibia 900. Thedevice 100 further comprises a spacer 120, which engages one or morearticular surfaces 820 of a distal portion 805 of the femur 800 and oneor more articular surfaces 920 of a proximal portion 905 of the tibia900. In this regard, the spacer 120 is adapted to at least partly alignin a coronal axis the tibia 900 and/or the femur 800 with a firstalignment axis (x). In the embodiment shown, the spacer 120 is integralwith the engaging portion 110 of the device 100.

In addition, the device 100 comprises guide apertures 112 and 114 forfacilitating the placement of one or more alignment pins or markers soas to facilitate resection of the femur 800 along a distal resectionplane (a). In this regard, these alignment pins or markers aresubsequently used to facilitate positioning of a distal femoralresection member on the femur 800. After positioning or placement of thedistal femoral resection member on the alignment pins or markers, thefemur 800 may be subsequently resected by way of resection apertures inthe distal femoral resection member that guide a resection tool alongthe distal resection plane (a).

As seen in FIGS. 10 and 11, the device 100 further includes guideapertures 116 and 118 for facilitating the placement of one or morealignment pins or markers so as to facilitate resection of the tibia 900along a proximal resection plane (b). Following their placement, aproximal tibial resection member can now be positioned or oriented onthe tibia 900 by way of sliding the resection member onto thesealignment pins or marks. After positioning or placement of the proximaltibial resection member on the alignment pins or markers, the tibia 900may be subsequently resected by way of resection apertures in theproximal tibial resection member that guide a resection tool along theproximal resection plane (b).

The embodiment of the device 100 shown in FIGS. 10 and 11 furthercomprises fixation apertures 130 and 135 for receiving fastening members1300 and 1400 respectively therethrough into femoral and tibial holes850 and 950 respectively so as to facilitate reversible fixation of thedevice 100 to the knee joint 700.

FIGS. 12 and 13 demonstrate an embodiment of a device 200 designed bythe method hereinbefore described to be used when the knee joint 700 isin flexion. The device 200 comprises an engaging portion 210 adapted toengage one or more articular surfaces 820 of a distal portion 805 of thefemur 800 and one or more anterior surfaces 910 of a proximal portion905 of the tibia 900. The device 200 further comprises a spacer 220,which engages one or more articular surfaces 820 and/or posteriorsurfaces 830 of a distal portion 805 of the femur 800 and one or morearticular surfaces 920 of a proximal portion 905 of the tibia 900. Inthe embodiment shown, the spacer 220 is engaging the medial articularsurfaces 835, 925 of the distal portion 805 of the femur 800 and theproximal portion 905 of the tibia 900 respectively. Further, the spacer120 is adapted to at least partly align in a coronal axis the tibia 900and/or the femur 800 with a second alignment axis (y). Similar to thatfor the device 100, the spacer 220 is integral with the engaging portion210 of the device 200.

The device 200 comprises guide apertures 211 and 212 for facilitatingthe placement of one or more alignment pins or markers in the femur soas to facilitate resection of the femur 800 along a posterior resectionplane (c). To this end, these alignment pins or markers are subsequentlyused to facilitate positioning or orientation of an anteroposteriorfemoral resection member on the femur 800, typically by sliding theresection member thereon. After positioning or placement of theanteroposterior femoral resection member on the alignment pins ormarkers, the femur 800 may be subsequently resected by way of resectionapertures in the anteroposterior femoral resection member that guide aresection tool along the posterior resection plane (c). Furthermore, theguide apertures 211 and 212 of the embodiment of the device in FIG. 12also mark the transepicondylar axis (TEA) (d) of the femur 800.

The device 200 further includes guide apertures 216 and 218, which,similar to that for guide apertures 116, 118 of the device 100,facilitate the placement or marking of one or more alignment pins ormarkers for facilitating resection of the tibia 900 along the proximalresection plane (b).

The embodiment of the device 200 shown in FIGS. 12 and 13 furthercomprises apertures 230, 232 and 235 for receiving fastening members1500, 1600 and 1700 respectively therethrough into femoral and tibialholes 860 and 960 respectively so as to facilitate reversible fixationof the device 200 to the knee joint 700.

FIGS. 14A and 14B illustrate a further embodiment of a patient-specificsurgical device 300 designed by the method hereinbefore described to beengaged with the femur 800. The patient-specific surgical device 300comprises an engaging portion 310 adapted to engage one or more anteriorsurfaces 810 and articular surfaces 820 of a distal portion 805 of thefemur 800.

The device 300 further comprises a resection aperture 312 for guiding aresection tool along a distal femoral resection plane (not shown). Inthis regard, the distal resection plane has been determined by thatdesign method hereinbefore described. Additionally, the device 300includes guide apertures 316 and 318 for facilitating the placement ofone or more alignment pins or markers so as to assist in marking thetransepicondylar axis of the femur 800 for subsequent placement orpositioning of a anteroposterior femoral resection member thereon toperform the anterior and/or posterior femoral resections.

As shown in FIGS. 14A and 14B device 300 further comprises fixationapertures 330 and 335 for receiving fastening members therethrough so asto facilitate reversible fixation of the device 300 to the femur 800.

FIGS. 15A and 15B illustrate a further embodiment of a patient-specificsurgical device 400 designed by the method hereinbefore described to beengaged with the tibia 900. The patient-specific surgical device 400comprises an engaging portion 410 adapted to engage one or more anteriorsurfaces 910 and articular surfaces 920 of a proximal portion 905 of thetibia 900.

As illustrated in FIGS. 15A and 15B, the device 400 includes a resectionaperture 412 for guiding a resection tool along a proximal tibialresection plane (not shown), which has been determined by that designmethod hereinbefore described. The device 400 further comprises fixationapertures 430, 431, 434, 434 and 435 for receiving fastening memberstherethrough so as to facilitate reversible fixation of the device 400to the tibia 900. The fixation apertures 430, 431 and 431, 432 mayfurther function to produce pinholes or markings on the superior surfaceof the tibia 900 so as to facilitate marking the anteroposterior andmediolateral position respectively of the tibia 900 relative to thefemur 800. Similarly, fixation apertures 434 and 435 may also functionto facilitate marking axial rotation of the tibia 900 relative to thefemur 800.

As would be readily understood by the skilled artisan, the dimensionsfor the device of the third aspect as well as the relative positions ofthe guide apertures and/or resection apertures therein will depend tosome degree on the size of the knee joint to which the devices are to beapplied.

Throughout the specification the aim has been to describe the preferredembodiments of the invention without limiting the invention to any oneembodiment or specific collection of features. It will therefore beappreciated by those of skill in the art that, in light of the instantdisclosure, various modifications and changes can be made in theparticular embodiments exemplified without departing from the scope ofthe present invention.

All computer programs, algorithms, patent and scientific literaturereferred to herein is incorporated herein by reference.

The following non-limiting examples illustrate the patient-specificsurgical devices and methods of the invention. These examples should notbe construed as limiting: the examples are included for the purposes ofillustration only. The patient-specific surgical devices and methodsdiscussed in the Examples 1-4 will be understood to represent anexemplification of the invention.

Example 1 Method

Patient-specific surgical devices designed as per Scheme A were appliedto the intact knee in extension and/or flexion at the start of a TKAprior to soft tissue releases to mark bone resections. ThePatient-specific surgical devices contacted the femur and tibiasimultaneously and acted to re-align the knee and tension theperi-articular soft tissues. Ten (10) patients had guides applied inflexion and extension and 3 additional patients used guides in extensiononly. A forty (40) patient cohort was similarly analysed. Data wasprospectively collected on these 13 patients examining medial andlateral soft tissue laxity in maximum extension, 20° and 90° of flexionpre and post-operatively.

Results

Values for periarticular tissue tension of the knee within normal rangesand minimal medial lateral and 0 vs 90° difference is provided in Table1 below. An appropriately balanced knee will typically meet all of thesecriteria.

TABLE 1 Normal ranges for periarticular tissue tension Medial vs Medial(°) Lateral (°) Lateral (°)  0° 1.5-7 2-8 ≤2 20°   0-6 0.5-9   ≤3 90°0.5-5 1-6 ≤3 0° vs 90° ≤2 ≤3 Medial and Lateral

Conventional Metal Jigs

Table 2 below demonstrates the balance achieved with respect to thenormal ranges in Table 1, in the first proof of concept study usingcomputer navigation and conventional metal resection jigs. As can beseen from this data, the balance is particularly good in extension andmedially which is advocated as being of primary importance.

This degree of balance was associated with high patient satisfactionbased on the validated satisfaction section of the American knee society(see FIG. 22). These results are equivalent to the best publishedresults for satisfaction seen in recent studies where force sensors wereused to measure pressure to document balance in a knee.

TABLE 2 Balance achieved versus normal values (Table 1) Medial vs Medial(°) Lateral (°) Lateral (°)  0° 98.3% 96.7% 93.4% 20° 93.4% 96.7% 90.2%90° 93.4% 55.7% 57.4% 0° vs 90° 91.8% 59.0% 59.0%

Ten Patient Cohort:

Soft tissue tension within parameters described for normal subjects wasattained medially and laterally in: 0 and 20° of flexion for all 13cases utilising the extension guide and in all 10 cases in 90° offlexion utilising the flexion guide.

Post-operatively the mean difference in medial versus lateral laxity was0.8° at 0°, 0.88° at 20° and 0.85° at 90° of flexion with a range of0-1.5° across all positions and all knees. Pre-operatively thedifference between medial and lateral laxity across all knees andassessed at each position tested ranged from 0-9°. Preoperatively themean difference in medial versus lateral laxity was 4.3° at 0°, 5.5° at20° and 1.58° at 90° of flexion.

In the ten cases utilising both flexion and extension guides, 0 versus90° flexion laxity on the medial side displayed a mean difference of1.4° (range 0-3) and on the lateral side a mean difference of 1.75°(range 0.5-4) was recorded.

Forty Patient Cohort

TABLE 3 Balance achieved versus normal values (Table 1) Medial (°)Lateral (°) Medial vs Lateral (°)  0°  95% 87.5% 97.5% 20° 100% 97.5%  90% 90°  95% 82.5%   85% 0° vs 90°  90% 82.5%   80%The data for the forty patient cohort also demonstrates a minimal sideto side difference in tissue laxity at different flexion points (seeTable 4). These are very similar results to those for the conventionalmetal jigs (see Table 2), except for an increase in normality of lateraltissues

TABLE 4 Medial vs lateral balance Difference Medial vs Lateral tissuelaxity Pre-op Post-op  0° 5.2° 0.79° 20° 5.81° 1.4° 90° 1.92° 1.51°Range all knees and 0-12.5° 0-4.5° positions

As shown in Table 5, this data seemingly closely approximates the normalsituation, such as that demonstrated by a recent cadaver study (i.e.,Roth et al.), which measured the difference in laxity in normal kneesbetween 0 and 90 degrees of flexion.

TABLE 5 Difference in laxity between 0 and 90 degrees of flexion Roth etal. Mean Mean Difference 0 Difference 0 vs 90° vs 90° tissue laxityRange tissue laxity Medial 1.13° 0.5-5.5°   1 ± 0.5 Lateral 2°   0.5-8°2.5 ± 0.8

In addition to the above, medial and lateral laxity in 0 and 90 degreesof flexion when compared against those values recommended by Hesterbeeket al. (see below) for TKA, demonstrate that our mean laxities sit closeto the mid point of the recommended ranges with very few readingsoutside the recommended range.

Hesterbeek recommended laxity values:

-   -   medial: 0.7-3.9° (extension); 0-5.5° (flexion)    -   lateral: 0.2-5.4° (extension); 0-7.1° (flexion)    -   Mean medial laxity extension −2.4° (range 1-5.5°) (1 reading        outside Hesterbeek range)    -   Mean medial laxity flexion −3.2° (range 1-8°) (2 readings        outside Hesterbeek range)    -   Mean lateral laxity extension −2.4° (range 1-4°)    -   Mean lateral laxity flexion −4.1° (range 0-10°) (3 readings        outside Hesterbeek range)

Conclusions

These patient-specific surgical devices show promise for attaining softtissue tension that is both normal and balanced across the knee joint.This may help diminish revision due to instability which was recorded asthe revision diagnosis in 18.7% of patients (Lombardi 2013).

Example 2

The primary purpose of the study was to determine whether the coronalplane tissue of the varus OA knee is contracted medially or laxlaterally. Our null hypothesis was that medial contractures would not bepresent in the varus OA knee when referenced to the corrected, neutralaxis of each knee.

Materials and Methods

Seventy-two Computer Assisted Surgery (CAS) TKA patients contributing 79knees were included in the study. Inclusion criteria were patients withvarus degenerative osteoarthritis of any degree scheduled for primaryTKA who had not undergone previous ligament reconstruction surgery, kneeosteotomy or suffered other trauma likely to distort the peri-articularsoft tissues. Exclusion criteria were patients who declined consent orwhere intra-operatively placement of navigation pins was considered highrisk due to poor bone stock or soft tissue. Ethical approval wasobtained from the relevant institutional review board and writteninformed consent was obtained pre-operatively for all patients.

A medial para-patellar approach to the knee was undertaken. Femoral andtibial navigation pins were inserted. All landmarks were identified andregistered in the navigation system for calculation of the mechanicalaxes of the femur, tibia, lower limb and generation of an individualised3D model of the patient's anatomy using computer navigation software(BrainLab, Munich, Germany). The status of the anterior cruciateligament (ACL) and posterior cruciate ligament (PCL) was recorded.Osteophytes were left in-situ to assess the knee as close to itspathological OA state as possible. The anterior horn of the medialmeniscus was left intact. If required release of the deep band of themedial collateral ligament was limited to 5 mm by sharp dissection inorder to maintain the state of medial soft tissues as close to thepre-operative state as possible whilst allowing clear navigationregistration similar to the methods of Bellemans et al [2].

The patella was reduced and the degree of fixed flexion deformity (FFD)or hyperextension was recorded in maximum extension. The mechanical axisof the limb was corrected to the centre of the knee by observing CASdisplays. The femoral and tibial mechanical axes were manipulated to beas co-linear as possible to the limb axes at this position. Directobservation of the joint was undertaken during measurements to preventsubluxation and ensure congruency (FIG. 16). These methods produced alimb position acting to define a neutral, corrected alignment for eachknee consistent with prior literature [6,7]. The Hip Knee Ankle Angle(HKAA) was recorded at this point. The knee was moved through amedial-lateral arc via manual force from this corrected position.Measurements were defined by medial and lateral deviation from thecorrected axis point in degrees as indicated by the navigation software.These measurements were undertaken in maximal extension and 20 degreesof flexion. The point of maximum varus deformity or the “loaded HKAA”was recorded.

Modern navigation systems have been validated for these measurements[2]. A medial coronal plane contracture was considered present when theknee could not be aligned to its neutral, corrected axis or when theknee did not correct beyond its axis displaying movement withinpreviously documented physiological ranges for elderly controls [8]. Inthese circumstances, medial laxity was recorded as zero. Lateral laxitywas considered abnormal when it exceeded 8° [2,8,9]. Once the data wasrecorded a CAS TKA was performed. In all cases, the procedure andmeasurements were performed by a single, high volume arthroplastysurgeon (MM) at a single centre.

All data was prepared and analysed using Prism 5 for Mac OS X Version 5(GraphPad Software Inc. La Jolla, Calif.). Statistical comparisons weremade using one-way analysis of variance with Tukey's Multiple Comparisontests.

Results

Seventy-two patients contributed 79 TKA's. Five males and two femalesunderwent bilateral TKA's. Table 6 demonstrates the demographics of thestudy population.

TABLE 6 Patient demographic detail. Mean +/− SD Range Age (years) 64.1+/− 7.2 49-81 Sex 40 Male, 32 Female Body Mass Index 32.3 +/− 5.320.2-52.2 (BMI) FFD (degrees)  5.0 +/− 5.2 18.5 to −8.5 (-vehyperextension) Varus (degrees) −7.8 +/− 3.1 −15 to −3

Seventy-nine knees were included in the study; however, 78 knees wereavailable to examine individual patterns of medial and lateral laxity.The reason for this was in one subject, the mechanical axis of the limbcould not be manipulated to pass through the centre of the knee toattain a corrected, neutral axis in maximum extension or 20° of flexion.Therefore, no zero point was attained and thus no medial and laterallaxity values were recorded. This subject was a 54-year-old female witha FFD of 0.5° and a varus deformity (loaded HKAA) of −13.5°. Thispatient was important to the study as a contracted knee. A medialcontracture was thus present in 13.9% (11/79) of knees. Table 7 displaysthe patterns of contracture and laxity in the remaining 78 patientscompared to published values in healthy knees.

TABLE 7 Distribution of Laxity Patterns in 78 subjects with individuallaxity assessment. Normal medial and lateral laxity was defined inrelation to published laxity data ranges for healthy subjects [2, 8, 9].Normal Medial laxity Medial contracture (≥0.5°) (<0.5°) Normal Laterallaxity 74.4% (58/78) 6.4% (5/78) (≤8°) Abnormally increased 12.8%(10/78) 6.4% (5/78) lateral laxity (>8°)

The majority of subjects (74.4%) recorded coronal laxity parameterswithin limits previously published for healthy knees. We then examinedthose subjects with medial contractures, abnormal lateral laxity and therelationship between varus deformity and soft tissue alteration in moredetail.

The neutral, corrected HKAA in the 10 patients with a medial contracturewho could be manipulated to but not beyond their corrected axis pointwithin physiological laxity ranges was −1.5+/−0.9°. There was nosignificant difference between the neutral, corrected HKAA of these 10contracted knees and the neutral, corrected HKAA of the 68/78non-contracted knees (p=0.819). A significant difference 2.3°±0.9°(p=0.015) was seen between the mean maximum varus (loaded axis) ofcontracted (−9.7°+/−0.8°, −14.5° to −6.5°) and non-contracted knees(−7.4°+/−0.3°; −15° to −2°).

In 20° of flexion a significant mean difference 2.2°±0.5° (p<0.001) inmedial laxity persisted between contracted and non-contracted knees. Twoof the ten contracted knees were still unable to be manipulated in 20°of flexion beyond their neutral, corrected HKAA within physiologicalranges. In these knees, medial laxity was recorded as zero.

Contracted knees were more lax laterally. A significantly increaseddifference in the mean lateral laxity of contracted versusnon-contracted knees in maximum extension 2.2°±0.8 (p=0.01), and 20° offlexion 2.4°±0.9 (p=0.008) was recorded.

TABLE 8 Laxity data in degrees for all knees, corrected and uncorrectedknees with comparison of contracted versus non-contracted knees. Non-Non-contracted All Varus contracted Contracted vs Contracted- Knees (78)Knees (68) Knees (10) p-value Medial 1.6 +/− 1.0 1.8 +/− 0.9 0 NALaxity-Max (0.5-4) Ext Medial 3.0 +/− 1.7 3.3 +/− 1.6 1.1 +/− 0.7 <0.001Laxity-20deg (0.5-7.5) (0-2) Lateral 6.1 +/− 2.5 5.9 +/− 2.4 8.1 +/− 2.40.010 Laxity-Max (1-11.5) (4.5-11) Ext Lateral 6.8 +/− 2.7 6.5 +/− 2.68.9 +/− 2.4 0.008 Laxity-20deg (2-13) (5.0-14.5) Laxity Arc-Max 7.7 +/−2.3 7.7 +/− 2.2 8.1 +/− 2.4 0.607 Ext (3.5-14) (4.5-11.0) LaxityArc-20deg 9.7 +/− 2.4 9.8 +/− 2.3 10.0 +/− 2.0  0.697 (6-16) (7-14.5)

We then examined the entire cohort for changes in medial and laterallaxity by grouping subjects according to their maximum varus deformity(loaded axis) and comparing laxity parameters between these groups.Medial laxity decreased as the varus deformity of the limb increased.FIG. 17 demonstrates a statistically significant mean difference inmedial laxity between 0-5° vs 5-10° but non-significant for 5-10°vs >10° in both maximum extension and 20° of flexion.

Lateral laxity increased as the varus deformity of the limb increased.There was a significant increase in laxity between each point ofincreasing varus deformity in both maximum extension and 20° of flexion(FIG. 18).

We analysed the soft tissue parameters in maximum extension and 20° offlexion to confirm that a true difference existed between these twomeasurement positions. We found the coronal soft tissues to have a totalmedial-lateral coronal arc at 20° of flexion) (9.7°±2.4° that wassignificantly greater (p<0.001) compared with the arc demonstrated atmaximum extension)(7.7°±2.3°. Both medial and lateral laxity for 20°flexion versus maximum extension was significantly increased with meandifferences of 1.4° (p<0.001) and 0.6° (p=0.007).

The study was primarily designed to define the incidence of medialcontracture and abnormal lateral laxity. Additionally we identifiedunexpected patterns of laxity in the study group that would potentiallyaffect surgery. These alterations in coronal laxity were not constantbetween maximum extension and 20° of flexion. More medial laxity thanlateral laxity at full extension was recorded in 6/78 (7.6%) knees. Fiveof these six knees maintained the same pattern at 20° of flexion, whilesix other knees displayed greater medial than lateral laxity at 20° offlexion. Therefore, at 20 degrees of flexion, 14.1% (11/78) of varusknees have greater medial than lateral laxity.

Discussion

The effect of OA on the soft tissues of the knee still requirescomprehensive definition. This information is important forintra-operative decision making during TKA. Studies to date haveexamined the coronal plane tissues of the knee in a single positionpreoperatively, varied positions intraoperatively after bony resectionor post-operatively without reference to initial soft tissue parameters[2,3,4,5,9,10,11,12]. The purpose of this study was to define thecoronal plane medial and lateral laxity of the end stage OA, knee priorto TKA, referenced from the corrected, neutral HKAA of each knee. Thiswas undertaken in maximum extension and 20° of flexion to assess thecontributions of posterior and coronal plane structures to anyalteration of tissue laxity. These measurements were compared topublished values for healthy subjects to assess the impact of OA on thecoronal plane soft tissues of the knee. Given this comparison, it isimportant to understand the measurement of medial and lateral laxity inthe healthy knee and how this corresponds to our study methods.

In the healthy knee, medial and lateral laxity is measured as deviationfrom the resting point of the knee. This point is not constant betweensubjects. This is seen in studies of healthy knees assessing theirresting or neutral position with mean HKAA values ranging between −1 to−1.3° and standard deviations in these studies between 2-2.8°[6,7,13,14,15,16]. The laxity measures documented when measuring fromthese resting positions in normal subjects are displayed in Table 9[8,9,17,18,19,20].

The individual initial neutral position of the OA knee and lower limb isunknown, due to its alteration by disease. This is important because ifwe cannot return a knee to its pre-disease position it is impossible tomeasure any medial and lateral distortion of the soft tissues by OA.Previous studies have used differing techniques to address this issue.Bellemans and Hohman et al both determined contracture to be presentbased on whether a knee could be realigned to a constant reference pointof a hip knee ankle angle (HKAA) of 0° and the correlation between thesubject's initial varus deformity and measures of maximal varus andvalgus stress alignment [2,3]. The healthy knee has a mean HKAA of−1.33°+/−2.34° [6]. The mean medial and lateral laxity in extension hasbeen reported in the elderly healthy knee to be 2.3+/−0.9° and2.8+/−1.3° respectively [8]. An osteoarthritic knee that had apre-disease HKAA of −4° (varus) is unlikely to realign to a HKAA of 0°whether or not a medial contracture is present. Therefore, whether avarus knee can attain a HKAA of 0° is not definitive evidence for thepresence or absence of a medial contracture. Similarly, the points ofmaximum varus or valgus attained under manual stress will be affected byinitial limb alignment.

Other studies have assessed the absolute gap with tensor devices afterbone resection, with or without the consideration of the amount of boneresected [4,5]. This approach measures gaps after alteration of both thebony and ligamentous anatomy of the knee and equalisation of pressure onthe medial and lateral sides of the knee. It does not account for theinitial position of the limb or the fact that medial and lateral laxityis typically different both in in-vivo studies and a recent cadaverstudy has confirmed these findings [8,9,17,18,19,20,21].

To address these concerns we placed the mechanical axis of the limbthrough the centre of the knee, whilst ensuring congruency of the jointunder direct vision and made the femoral and tibial mechanical axes asco-linear as possible. This created a corrected or neutral HKAA positionfor each knee that is consistent with prior literature and thus allowedthe measurement of laxity to be individualised for each subject [6,16].It is important to note a HKAA of 0° will only occur when the tibial andfemoral axes match at the knee and they are both co-linear with themechanical axis of the limb. More commonly, the neutral HKAA willapproach 0° (FIG. 1). In our subjects, the mean neutral, corrected HKAAin in maximum extension was −1.4+/−1.3 with a range of 1.5 to −5.5. Thisis similar to the mean HKAA and the varus range in the healthypopulation [6]. We believe these factors mean our neutral point fromwhich we measured medial and lateral laxity in each knee is a closeapproximation of the pre-disease alignment of that knee and addressesmany of the issues demonstrated in previous studies that have sought toexamine the alteration of coronal plane knee tissues by OA [2,3,4,5].

Our results have demonstrated the variable nature of laxity patterns inthe coronal plane of the varus OA knee with up to 15° varus deformity.Our findings support both the findings of contracture and laxity byprevious authors but place them in a more individual context and helpreconcile the conflicting prior findings in the orthopaedic literature[2,3,4,5]. The “classic” pattern of contracted medial tissue and laxlateral tissue was only recorded in 6.4% (5/78) of patients. Medialcontracture and lateral laxity also exist in isolation with normaltissue laxity found on the opposite side of the joint in many patients.The majority of patients in this study had medial and lateral laxitymeasurements within ranges previously described for healthy subjects[8,17,18,19,20].

We found a medial contracture to be present in 12.8% (10/78) of patientsin maximum extension. Hohman and Bellemans et al found contracturesdeveloped at varus deformity beyond ten degrees. In our subjects, kneeswith contracture had a significantly higher mean maximum deformity(loaded axis) value than non-contracted knees but displayed a range ofmaximum deformity of −6.5 to −14.5°. The initial alignment from whichthese knees developed this deformity and contracture did not appear tobe different between contracted and non-contracted knees with nosignificant difference found between the neutral corrected HKAA of thesetwo groups. In the cohort as a whole we found a non-significant decreasein medial laxity comparing subjects with 5-10° of deformity to >10° ofdeformity. Therefore, increased varus deformity is often present withcontracture but deformity of >10° is not a reliable proxy marker for thepresence of contracture, indeed the mean varus deformity of contractedknees in our series was 9.7°. When contractures are present they haveboth a posterior and medial contribution as demonstrated by thesignificant difference in medial laxity between contracted andnon-contracted knees at both maximum extension and 20° of flexion. Theinfluence of the posterior structures is also demonstrated by the factthat of the eleven subjects with contracture in maximum extension onlythree met the same criteria for contracture in 20° of flexion.Additionally, significant increases in medial and lateral laxity, andtotal coronal motion were seen in 20° of flexion once the influence ofthe posterior structures was removed.

The mean medial laxity in our subjects at maximum extension with was1.6+/−1.0° this compares to the healthy population where mean mediallaxity is reported to range between 2.3-3.6° [8,9,18]. The smalldifferences in results may reflect variations in subject groups andinvestigative techniques but in any case, these differences in mediallaxity are likely below or at the margin of clinical significance.

These findings have disproved our null hypothesis that medialcontracture is not present in the varus OA knee when referenced to thecorrected, neutral HKAA of each knee.

Studies of healthy knees have shown mean lateral laxity in eithermaximum extension or 10° of flexion varying between 4.1-4.9° [8,9,18].In a study utilising similar investigative techniques to ours Jennyfound a range for lateral laxity in maximum extension in healthysubjects of 2-8°, similar findings are seen in other studies [2,8,9].Our subjects had a mean lateral laxity of 6.1+/−2.5°. In our subjects,19.2% (15/78) of knees had lateral laxity of more than 8° (8.5°-11.5°)in maximum extension. We found a significant increase in laxity as varusdeformity increased. Therefore, we would agree with the findings ofOkamoto et al who described lateral laxity in the varus knee with moresevere deformity [3.] We have been able to better quantify theindividual degree and patterns of increased laxity showing it can existin isolation or combined with a medial contracture.

Counter intuitively given the varus alignment of our subjects, 6/79(7.6%) knees in extension and 11/79 (13.9%) knees in 20° of flexionshowed an unexpected pattern of increased medial versus lateral laxitywhich has not been previously reported.

The findings of these laxity patterns in our subjects have directrelevance to TKA. Surgeons should not assume laxity patterns based ondeformity or that abnormality in the coronal plane of the knee on oneside of the joint correlates to abnormality on the opposite side of thejoint. The “releases” that occur as part of the approach to the kneeduring TKA and resection of osteophytes will likely address the degreeof medial contracture we have demonstrated in the majority of cases witha varus deformity of up to 15°. Therefore, if an extensive medialrelease is required it may represent an uncommon clinical situation orimportantly reflect errors within the surgical procedure. If acontracture is present then both a medial and posterior cause should besought. In patients with abnormal lateral laxity the surgeon should notrelease medial tissue to attain balance based on abnormal lateral tissueand careful surgical technique should avoid any further increase inlateral laxity to potentially problematic levels [22].

We would acknowledge the limitations of our study. The patient group waspredominantly Caucasian and their selection for TKA may not beindicative of other surgical series. Our study is consistent withdescribed in-vivo techniques for measuring coronal plane laxity and oursubjects had flexion and varus deformity parameters similar topreviously examined OA knee groups in the literature[2,3,6,7,8,17,18,23,24]. Many studies define coronal plane knee laxityby a total arc of movement at 20° of flexion and our findings areconsistent these prior studies [17,23,24]. Therefore, we believe thefindings of our study are valid in the OA knee with varus deformity ofup to 15°.

Manual stress testing was performed to assess coronal plane laxity. Thisis consistent with prior studies [2,9]. Whilst an experienced surgeonperformed this, some degree of variability in the forces exerted duringtesting is inevitable. Mitigating this is the fact that ligaments areviscoelastic structures and measurements were recorded at maximaldisplacement. This should correspond to the plateau of thetension/length curve minimising any effects of variation in force [9].It is not possible in an in-vivo setting to utilise the same invasivetechniques with multiple controlled parameters and measurements that areutilised in cadaver studies given the ethical implications of prolongingsurgery particularly in relation to infection and fracture risk [21]. Itis also possible to take a pragmatic view of our study given that ourlaxities were measured at a point in the surgical exposure less thanthat required to complete the operation. Therefore, the measuredlaxities approximate those present at a point near the commencement ofsurgery and thus our results are useful for planning surgery from thisinitial point even if they do not represent the pure measure of medialand lateral laxity in the in-vivo setting of the end stage OA knee.

We cannot know that the corrected, neutral HKAA was the best startingpoint from which to measure medial and lateral coronal laxity but we donot believe it would have altered the conclusions of our study. Ourcorrected, neutral HKAA range of 1.5 to −5.5° is consistent with thevarus HKAA range demonstrated by Bellemans et al in the healthypopulation of 0° to −8° [6]. Our measurements were non-weight bearing.Studies show increased varus alignment with weight bearing of 1.6-2°[25,26]. An increase in varus alignment would have acted to furthernormalise our HKAA range and medialize the initial corrected, neutralpoint. The effect of measuring displacement from a more varus startingposition would have been to increase the medial and decrease the laterallaxity measured further normalising our results.

In conclusion we have shown medial contractures referenced to theindividual corrected, neutral axis of the varus OA knee with up to 15°deformity are uncommon. When present they are relatively small and occurvia contributions from both medial and posterior structures. Increasedlateral laxity is a more common finding but the patterns of coronalplane laxity alteration in the end stage OA knee at the time of TKA arehighly variable and correlate poorly to the initial or subsequent varusdeformity of the limb. Our findings help reconcile the previous conflictin the orthopaedic literature. An awareness of these findings willassist the surgeon during TKA to optimally balance and normalise thecoronal plane soft tissues.

TABLE 9 Literature review of in-vivo coronal plane laxity in the normaland OA knee. Mean with standard deviation data (+/− range). Med- Med-Med- Med- Med- Med- Med- Med- Med- Med- Med- Med- Num- Mea- ial ial ialial ial ial ial ial ial ial ial ial ber sure- Lax- Lax- Lax- Lax- Lax-Lax- Lax- Lax- Lax- Lax- Lax- Lax- of ment ity- ity- ity- ity- ity- ity-ity- ity- ity- ity- ity- ity- Popu- Kne- Tech- 0 10 20 70 80 90 0 10 2070 80 90 Paper lation es Age nique deg deg deg deg deg deg deg deg degdeg deg deg Oka- Healthy 50 25.9 Stress 2.4 1.7 4.9 4.8 zaki +/− X-ray+/− +/− +/− +/− (2005) 7.5 1.8 1.4 2.0 3.2 Haes- Heathy 10 62 Stress 2.32.5 2.8 3.1 ter- (6.4) X-ray +/− +/− +/− +/−2.0 beek 0.9 1.5 1.3 (range(2008) (range (range (range 0.1-7) 0.1- 0-6.0) 0.6- 1.1) 5.4) JennyHealthy 20 24 Com- 3.6 2.1 4.1 3.7 (2009) (18- puter +/− +/− +/− +/− 36)assign- 1.2 1.2 1.9 1.2 ed (range (range (range (range sur- 2-7) 0-5)2-8) 2-0) gery Creaby Healthy 32 59 Dyna- 8.4 10.7 (2010) mo- +/− +/−meter 3.2 3.7 Toko- Healthy 20 27.2 MRI 2.8 7.9 hura (18- leg +/− +/−(2004) 53) weight 0.9 1.9 Belle- Varus 35 — Com- 0.6 4.1 maus OA puter+/− (range (2009) assign- (range 2-8) ed 3-4) sur- gery Creaby Varus 3266 Dyna- 9.2 8.5 (2010) OA mo- +/− +/− meter 2.9 2.7 Mc- Varus 55 64Com- 1.6 3.1 3.9 6.2 6.9 4.7 Auliffe OA (49- puter +/− +/− +/− +/− +/−+/− (2015) 79) assign- 1.1 1.7 1.4 2.8 2.9 2.0 ed (0-4) (0- (1- (0-12)(2-15) (1-12) sur- 7.5) 7.5) gery

REFERENCES

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Example 3

The purpose of our study was to use computer navigation to correct theHKAA in maximum extension to a neutral alignment for that limb prior toTKA and compare this data to normative HKAA values and examine whetherthis could focus the current 6° alignment range for each knee.^(4,10-13)Our hypothesis was that the corrected, neutral HKAA of limbs undergoingTKA was not significantly different to the HKAA of healthy knees andthat this method of individualizing coronal plane alignment may beuseful in optimizing coronal alignment goals for TKA.

Materials and Methods

80 Computer Assisted Surgery (CAS) TKA patients contributing 89 kneeswere included in the study. These patients were part of a broader studyexamining the peri-articular soft tissue envelope at the time of TKA.Inclusion criteria were: patients with degenerative osteoarthritisscheduled for primary TKA who had not undergone previous ligamentreconstruction surgery; knee osteotomy or suffered other trauma likelyto distort the peri-articular soft tissues or the overall alignment ofthe limb. Exclusion criteria were: patients who declined to consent toparticipation or where intra-operatively placement of navigation pinswas considered high risk due to bone stock or poor soft tissue quality.Ethical approval was attained from the relevant institutional reviewboard and written informed consent was obtained pre-operatively for allpatients.

A medial para-patellar approach to the knee was undertaken. Femoral andtibial navigation pins were inserted. All landmarks were identified andregistered in the navigation system for calculation of the mechanicalaxes of the femur, tibia, lower limb and generation of an individualised3D model of the patient's anatomy using computer navigation software(BrainLab, Munich, Germany). The patella was reduced and the degree offixed flexion deformity (FFD) or hyperextension was recorded in maximumextension. The mechanical axis of the limb was corrected in maximumextension by manipulating the knee, while observing CAS displays anddirect observation of the joint to prevent subluxation and ensurecongruency. These methods produced a limb position from which wemeasured a neutral or corrected HKAA for each knee consistent with priorliterature.^(3,4) FIG. 16 demonstrates this correction.

Limbs were also loaded manually to exacerbate their initial deformity toa maximum varus or valgus position. This produced a “loaded HKAA” ormaximum point of deformity, which was also recorded. In all cases, theprocedure and measurements were performed by a single, high volumearthroplasty surgeon (MM) at a single centre.

All data was prepared and analysed using Prism 5 for Mac OS X Version 5(GraphPad Software Inc. La Jolla, Calif.).

Results

80 patients contributed 89 TKA's, six male subjects and three femalesubjects underwent bilateral TKA. The demographics of the studypopulation are shown in Table 10.

TABLE 10 Patient demographic detail. Negative FFD denoteshyperextension. Mean +/− SD Range Age (years) 63.8 +/− 7.1 49-81 Sex 39Male, 41 Female Body Mass Index 32.1 +/− 5.2 20.2-52.2 (BMI) FFD(degrees)  4.5 +/− 4.9 −8.5-15   

The loaded and corrected HKAA values for all varus and valgus knees arepresented in Table 11. There was no significant difference in theabsolute change in HKAA between loaded and corrected axes when comparingvarus to valgus knees (p=0.40). A significant difference was seen invarus and valgus knees with respect to FFD (p=0.02)

TABLE 11 Analysis of the corrected and loaded HKAA in extension for theentire cohort and varus and valgus knees. Overall Varus knees Valgusknees (n = 89) (n = 72) (n = 17) Loaded HKA (degrees) −5.1 +/− 6.7 −7.9+/− 3.3 6.9 +/− 2.9 (−20 to 13.5) (−20 to −0.5) (2.5 to 13.5) CorrectedHKA Angle −1.2 +/− 1.4 −1.5 +/− 1.3 0.0 +/− 1.0 (degrees)-max. ext.(−5.5 to 2) (−5.5 to 1) (−1 to 2) Absolute change in N/A   6.3 +/− 3.17.0 +/− 2.8 HKA axis between (0 to 17.5) (2 to 12.5) loaded andcorrected axes (degrees) for varus and valgus knees FFD (degrees)   4.5+/− 4.9   5.1 +/− 5.1 2.0 +/− 3.3 (negative FFD denotes (15 to −8.5) (15to −8.5) (−5.5 to 6) hyperextension)All values are in degrees and expressed as Mean+/−SD (Range). Negativedenotes varus or hyperextension.

FIG. 19 depicts the alteration from the loaded HKAA to the correctedHKAA. The corrected HKAA in maximum extension was within +/−3° of 0° in91.0% of patients. All valgus knees corrected to +/−3°. Eight varusknees did not display a corrected HKAA within this range. They recordeda corrected HKAA between −3.5° and −5.5°.

DISCUSSION

In TKA, a HKAA of 0° is often referred to as neutral mechanicalalignment. It is important to understand that the neutral axis of a limbis not necessarily reflected by a HKAA of 0° although in some knees theneutral axis will also be 0°. This is seen in studies of healthy kneesassessing their resting or neutral position where only a minority ofknees record a HKAA of 0° with mean values ranging between −1°-−1.3° andstandard deviations in these studies between 2°-2.8°. These studiesmeasure the HKAA in maximum extension but do not quantify the exactposition of flexion or extension in their subjects. Cooke et al note theexclusion of patients with radiographs taken in greater than 20° offlexion.^(4,10-13) A limb axis of 0° will only occur when the centre ofthe tibia and femur match at the level of the knee joint and bony andsoft tissue structures allow co-linearity of both the tibial and femoralmechanical axes. More typically, the axes approach co-linearity and asmall degree of varus or valgus will be recorded. By placing themechanical axis of the limb through the centre of the knee, ensuringcongruency of the joint under direct vision and making the femoral andtibial mechanical axes as co-linear as possible we have created acorrected HKAA measurement for each knee that is consistent with priorliterature.^(3,4) Modern navigation systems have been validated formeasurements of alignment with an accuracy of 0.5°-1.0° depending on thelevel of deformity.¹⁵

In the non-arthritic knee the mean HKAA has been reported by Bellemanset al as −1.33°+1-2.34°.⁴ The HKAA measured in our study was−1.2°+/−1.4° (Range: −5.5° to) 2°. There is no significant differencebetween these values (p=0.61). The values we recorded displayed a lowerSD. Our mean HKAA is consistent with the HKAA recorded in other studiesof healthy populations.^(4,10-13) It is likely we have idealised theHKAA by removing the influence of weight bearing and hindfoot alignment.Previous studies have found weight bearing increases varus mal-alignmentby a mean 1.6°-2° in varus knees.^(16,17) In addition, we found 91.0% ofknees aligned to +/−3° and the remaining knees displayed a neutral HKAAbetween −3° and −5.5°. Therefore, it was possible to place kneesundergoing TKA in an individualised neutral alignment either withinaccepted current alignment parameters of 0+/−3° or within 2.5° of theseparameters in 100% of cases.

This has confirmed our hypothesis that the neutral HKAA of limbsundergoing TKA is not significantly different to the HKAA of normalknees unaffected by OA. It also suggests that this method ofindividualizing coronal plane alignment of the knee may be useful inTKA. A TKA aligned within the current safe zone of +/−3° may besignificantly altered from its individual neutral alignment. TKA isnon-anatomic and results in at least some distortion of theperi-articular soft tissue envelope. It is important the selectedcoronal alignment does not further heighten this disturbance. Dependingon the degree of alteration, this may cause distortion or requirerelease of the coronal plane soft tissues, which have, mean medial andlateral values in healthy subjects of 2.3°+/−0.9° and 2.8°+/−1.3°respectively.⁵ Our results regarding the individualised neutralalignment of a knee would indicate that 91.0% of prostheses could beplaced both in an alignment that would not exceed the mechanicalproperties of a total knee prosthesis nor cause significant distortionof the coronal plane soft tissues of the knee. It may be appropriate toalign knees to >3° of varus to encompass the knees in our study wheretheir neutral alignment was found to be between −3°-−5.5° as there issupport, though not universal, for this alignment not alteringprosthetic survivorship.¹⁸⁻²¹ Alternatively, these knees could bealigned to the limit of the −3° range, which would result in less softtissue distortion whilst still respecting the mechanical limits of theprosthesis and neutral mechanical alignment goals. If we are to placeprostheses at the limit of current mechanical alignment parameters, thisrequires the development of consistently accurate and reliable surgicaltechniques that prevent further unintended coronal plane alignmentdeviations. These deviations have been documented in some cases withcurrent surgical techniques and this may compromise the longevity of theprosthesis.²²

Regardless of the theoretical advantages or disadvantages of deviatingfrom 0° of mechanical limb alignment we found that varus and valgusknees in our subjects did not show neutral alignment at the opposite endof the +/−3° range. Therefore, we believe optimal coronal plane TKAalignment might be better and safely redefined to −1° to 3° for a valgusknee and 1.5° to −3° for a varus knee and focussed on the correctedneutral axis of the limb.

We would acknowledge our study has limitations. We have analysed dataattained from CAS navigation techniques which is highly accurate formeasurement but may not be identical to HKAA values recorded on long legweight bearing radiographs in other studies.^(4,10-13) Our patient groupis reflective of the practice of a single surgeon and therefore may notbe applicable broadly to patients undergoing TKA, however, given thesimilarity in the coronal and flexion deformity parameters for oursubjects compared to other studies we believe our results are a validcontribution to the literature.^(4,13,25) We acknowledge that there isno way of knowing the original position of the lower limb prior to itsalteration by OA and passage to TKA. However, we are unlikely to everdefinitively answer this question given the logistics associated withsuch a study. Weight bearing is likely to have increased the spread ofour results but is not likely to have altered the conclusions of ourstudy.^(16,17) We have only examined static not dynamic alignment orother factors which are known to be important to OA progression.²⁴Whether the suggested coronal plane position is more optimal for anyindividual knee depends on the laxity found in the coronal plane tissuesat this point. This was beyond the scope of this paper but is an area weintend to study in the future. Despite these limitations, we feel theconclusions for this patient group are broadly applicable to the OA kneepopulation undergoing TKA.

In conclusion, we have demonstrated that the neutral HKAA of knees with+13.5° to −20° of deformity at the time of TKA is similar to the healthypopulation. Therefore, whilst alignment does have an influence on OAdevelopment, initial alignment does not appear to have a strongassociation with end stage knee OA requiring TKA. Varus and valgus kneesdo not have a neutral HKAA at the opposite end of the 0+/3° range. Ourmethod of planning coronal alignment may help lessen the inevitabledistortion of the peri-articular coronal plane soft tissues by TKA byproviding more focussed alignment goals.

REFERENCES

-   1. Howell S M, Hodapp E E, Vernace J V, Hull M L, Meade T D. Are    Undesirable Contact Kinematics Minimised After Kinematically Aligned    Total Knee Arthroplasty? An Intersurgeon Analysis of Consecutive    Patients. Knee Surg Sports Traumatol Arthrosc 2013; 21:2281-2287.-   2. Abdel M P, Oussedik S, Parratte S, Lustig S, Haddad F S. Coronal    alignment in Total Knee Replacement. Historical review, contemporary    analysis, and future direction. Bone Joint J 2014; 96-B(7):857-862.-   3. Cooke TDV, Sled E A, Scudamore R A. Frontal plane knee alignment:    a call for standardized measurement. J Rheumatol 2007; 34:1796-1801.-   4. Bellemans J, Colyn W, Vandenneucker H, Victor J. Is neutral    mechanical alignment normal for all patients? The concept of    constitutional varus. Clin Orthop Relat Res 2012 January;    470(1):45-53.-   5. Heesterbeek PJC, Verdonschot N, Wymenga A B. In Vivo Knee Laxity    in Flexion and Extension: A Radiographic Study in 30 Older Healthy    Subjects. The Knee 2008; 15:45-49.-   6. Hunter D J, Niu J, Felson D T, Harvey W F, Gross K D, McCree P et    al. Knee alignment does not predict incident osteoarthritis. The    Framingham Osteoarthritis Study. Arthritis and Rheumatism 2007    April; 56(4):1212-1218.-   7. Brouwer G M, van Tol A W, Bergink A P, Belo R M, Bernsen M D,    Reijman M et al. Association between Valgus and Varus Alignment and    the Development and Progression of Radiographic Osteoarthritis of    the Knee. Arthritis and Rheumatism 2007 April; 56(4): 1204-1211.-   8. Sharma L, Chmiel J S, Almagor O, Felson D, Guermazi A, Roemer F.    The role of varus and valgus alignment in the initial development of    knee cartilage damage by MRI: the MOST study. Ann Rheum Dis 2013;    72:235-240.-   9. Belo J N, Berger M Y, Reijman M, Koes B W, Bierma-Zeinstra SMA.    Prognostic factors of progression of osteoarthritis of the knee: A    systematic review of observational studies. Arthritis and Rheumatism    2007 February; 57(1):13-26.-   10. Chao EYS, Neluheni EVD, Hsu RWW, Paley D. Biomechanics of    malalignment. Orthopedic Clinics of North America 1994 July; 25 (3):    379-386.-   11. Hsu RWW, Himeno S, Coventry M B, Chao EYS. Normal axial    alignment of the lower extremity and load-bearing distribution at    the knee. Clinical Orthopaedics and Related Research 1990 June;    255:215-227.-   12. Moreland J R, Bassett L W, Hanker G J. Radiographic analysis of    the axial alignment of the lower extremity. J Bone Joint Surg Am    1987 June; 69(5):745-749.-   13. Cooke D, Scudamore A, Li J, Wyss U, Bryant T, Costigan P. Axial    lower limb alignment: comparison of knee geometry in normal    volunteers and osteoarthritic patients. Osteoarthritis and Cartilage    1997; 5:39-47.-   14. Cerejo R, Dunlop D D, Cahue S, Channin D, Song J, Sharma L. The    influence of alignment on risk of knee osteoarthritis progression    according to baseline stage of disease. Arthritis and Rheumatism    2002 October; 46(10):2632-2636.-   15. Bellemans J, Vandenneucker H, Vanlauwe J, Victor J. The    influence of coronal plane deformity on mediolateral ligament    status: an observational study in varus knees. Knee Surg Sports    Traumatol Arthrosc 2010; 18:152-156.-   16. Specogna A V, Birmingham T B, Hunt M A, Jones I C, Jenkyn T R,    Fowler P J et al. Radiographic measures of knee alignment in    patients with varus gonarthrosis: effect of weightbearing status and    associations with dynamic joint load. Am J Sports Med 2007; 1:65-70.-   17. Brouwer R W. Unicompartmental Osteoarthritis of the Knee:    Diagnosis and treatment of malalignment. Proefschrift    Rotterdam; 2006. ISBN 90-75092-47-4-   18. Parratte S, Pagnan M W, Trousdale R T, Berry D J. Effect of    postoperative mechanical axis alignment on the fifteen-year    survivial of modern, cemented total knee replacements. J Bone Joint    Surg Am 2010; 92-A:2143-2149.-   19. Matziolis G, Krocker D, Weiss U, Tohtz S, Perka C. A    Prospective, randomised study of computer assisted and conventional    total knee arthroplasty. three dimensional evaluation of implant    alignment and rotation. J Bone Joint Surg Am 2007; 89-A:236-243.-   20. Bonner T J, Eardley W G, Patterson P, Gregg P J. The effect of    post-operative mechanical alignment on the survival of primary total    knee replacements after a follow-up of 15 years. J Bone Joint Surg    Br 2011; 93-B:1217-1222.-   21. Collier M B, Engh C A Jr, McAuley J P, Engh G A. Factors    associated with the loss of thickness of polyethylene tibial    bearings after knee arthroplasty. J Bone Joint Surg Am 2007;    89-A:1306-1314.-   22. Luyckx T, Vanhoorebeeck F, Bellemans J. Should we aim at    undercorrection when doing a total knee arthroplasty? Knee Surg    Sports Trauma Arthrosc 2015 June; 23(6):1706-1712.-   23. Sharma L, Song J, Dunlop D, Felson D, Lewis C E, Segal N et al.    Varus and valgus alignment and incident and progressive knee    osteoarthritis. Ann Rheum Dis 2010; 69:1940-1945.-   24. Miyazaki T, Wada M, Kawahara H, Sato M, Baba H, Shimada S.    Dynamic load at baseline can predict radiographic disease    progression in medial compartment knee osteoarthritis. Ann Rheum Dis    2002; 61:617-622.-   25 Hohman D W, Nodzo S R, Phillips M, Fitz W. The implications of    mechanical alignment on soft tissue balancing in total knee    arthroplasty. Knee Surg Sports Trauma Arthrosc 2014 Sep. 13 [Epub    ahead of print].

Example 4

The primary purpose of this study was to define the medial-laterallaxity of the soft tissues of the varus OA knee in 90° of flexion priorto any surgical releases at the time of TKA.

Materials and Methods

Sixty-five Computer Assisted Surgery (CAS) TKA patients contributed 72knees, five males and two females had bilateral TKA's. Inclusioncriteria were patients with varus degenerative osteoarthritis of anydegree scheduled for primary TKA who had not undergone previous ligamentreconstruction surgery, knee osteotomy or suffered other trauma likelyto distort the peri-articular soft tissues. Exclusion criteria werepatients who declined to consent to participation or whereintra-operatively placement of navigation pins was considered high riskdue to poor bone stock or soft tissue. Ethical approval was attainedfrom the relevant institutional review board and written informedconsent was obtained pre-operatively for all patients.

A medial para-patellar approach to the knee was undertaken. Femoral andtibial navigation pins were inserted. All landmarks were identified andregistered in the navigation system for calculation of the mechanicalaxes of the femur, tibia, lower limb and generation of an individualised3D model of the patient's anatomy using computer navigation software(BrainLab, Munich, Germany). The status of the anterior cruciateligament (ACL) and posterior cruciate ligament (PCL) was recorded.Osteophytes were left in-situ to assess the knee as close to itspathological OA state as possible. The anterior horn of the medialmeniscus was left intact. Release of the deep band of the medialcollateral ligament was left intact or limited to 5 mm by sharpdissection in order to maintain the state of medial soft tissues asclose to the pre-operative state as possible whilst allowing clearnavigation registration similar to the methods of Bellemans et al [5].

The patella was reduced and the degree of Fixed Flexion Deformity (FFD)or hyperextension was recorded in maximum extension. The mechanical axisof the limb was corrected, in maximal extension and 20° of flexion, bymanipulating the knee while observing CAS displays. Direct observationof the joint was undertaken during this process to prevent subluxationand ensure congruency. These methods produced a limb position acting todefine a neutral alignment for the knee consistent with prior literature[12,13]. The knee was placed through a medial lateral arc with manualforce from this corrected position. Laxity measurements were defined bymedial and lateral displacement from the corrected axis point indegrees. These measurements were undertaken in maximal extension 93 and20 degrees of flexion.

In 90° of flexion, neutral rotation and co-linearity of the femoral,tibial and limb axes was attained via CAS displays with observation ofthe joint again used to ensure congruency. Laxity was measured as medialand lateral displacement in degrees from this neutral point via theweight of the limb and manual force consistent with prior techniques[9,10].

Modern navigation systems have been validated for these measurements[5]. In all cases, a single, high volume arthroplasty surgeon (MM) at asingle centre performed the procedure and measurements. A CAS TKA wasundertaken and the same measurements were repeated at the end of theprocedure.

All data was prepared and analysed using Prism 5 for Mac OS X Version 5(GraphPad Software Inc. La Jolla, Calif.). Direct statisticalcomparisons were made via Student's t-test. Any multiple comparisontesting was completed using Tukey's Multiple Comparison Test.

Results

Sixty-five patients contributed 72 TKA's, five male and two femalesubjects had bilateral TKA's. The demographics of the study populationare shown in Table 12.

Table 13 displays the results for medial and lateral laxity at eachmeasurement position. Our results demonstrated a significant, thoughdiminishing, difference between medial and lateral laxity at maximumextension p<0.001 (Mean −4.9°; 95% CI: −5.4° to −3.9°), 20° flexionp<0.001 (Mean −4.6°; 95% CI: −4.9° 115 to −2.9°) and 90° flexion p<0.001(Mean −1.5°; 95% CI: −1.3° to)−0.5°. Maximum extension versus 90°flexion laxity was significantly different on both the medial p<0.001(Mean −2.2°; 95% CI: −3.2° to)−1.3° and lateral sides p<0.001 (Mean1.5°; 95% CI: 0.6° to 2.5°).

The total medial-lateral coronal arcs were not significantly differentat maximum extension and 90° of flexion. The total medial-lateralcoronal arc at 20° of flexion was significantly greater compared withthe arcs demonstrated at maximum extension (p<0.001; Mean −2.2°; 95% CI:−2.9° to)−1.4° and 90° flexion (p<0.001; Mean 1.5°, 95% CI: 0.7° to2.2°).

The mean lateral minus medial laxity difference in 90° of flexion was1.5°+/−1.1° (0°-8°). Sixty-six (91.6%) knees had a difference of ≤2.5°;3 (4.1%) knees a difference of 3°; 2 (2.7%) knees 3.5° and 1 (1.4%) kneean 8° difference.

Discussion

The improved outcomes associated with attaining a balanced knee post TKAhave been documented [1,2]. If we can better understand the soft tissueenvelope at the commencement of TKA then we can plan surgery to preserveor correct the soft tissue envelope. Current literature has examinedin-vivo knee coronal plane laxity in different positions of flexion withdiffering investigative techniques and different patient groups. (Table9 above) [5-10,14-20]. This makes direct comparison of studiesdifficult. To our knowledge, the OA knee has not been examined in-vivoin 90° of flexion.

Our study is consistent with previously described measurement andsurgical techniques and patient groups including flexion and varusdeformity parameters [5,7,9,10,12,13,17,18,22]. We undertook laxitymeasurements with a reduced patella, which improves the accuracy ofthese measurements [21]. Arthrotomy of the knee does not significantlyalter the medial-lateral laxity of the knee [16]. Firstly, to confirmour protocols produced broadly valid measurements we compared ourresults to prior in-vivo studies.

Many studies define coronal plane knee laxity by a total arc of movementat 20° of flexion. FIG. 21 compares these studies [14,15,18]. Our studyis the largest varus OA group. It represents the first data for thisparameter specifically documented at the time of TKA and demonstratesresults consistent with these studies. Therefore, we believe thefindings of our study are a valid contribution to the in-vivo literaturein varus deformity of up to 15°, which was the maximum deformity in thisseries of subjects.

The purpose of our study was to define the medial and lateral laxity ofthe OA knee at the time of TKA in 90° of flexion and examine howosteoarthritis has altered it compared to the healthy knee. The studiesof Brage, Wada and Creaby all demonstrate a non-significant differencebetween healthy and OA knees tested under the same conditions at 20° offlexion [14,15,18]. The alteration if any of the coronal plane tissue inthe OA knee versus the healthy knee in 90° of flexion is unknown. Thehealthy knee displays different laxity parameters at 70-90° of flexioncompared to extension and similar findings are present in a recentcadaver study [7,8,9,11].

Coronal plane laxity at 90° of flexion is important to the outcome of aTKA. Studies have examined the end result of surgery but not the initialsoft tissue envelope at the time of TKA [2,23,24,25]. Whilst coronalplane tissue laxity in the OA knee at 90° of flexion remains unknown, itis uncertain whether TKA should reproduce or correct this laxity.

Our study has shown medial and lateral laxity measured from the neutralaxis in 90° flexion to be 3.8°+/−1.4° and 4.7°+/−2° respectively. Onlytwo studies have specifically examined medial and lateral laxity in thehealthy knee in-vivo at 90° of flexion [9,10]. Of these studies, Jennyhas employed similar investigative techniques to our series in studying20 normal knees at the time of anterior cruciate ligament reconstruction(ACLR) with computer navigation [9]. Whilst the knees of Jenny were ACLdeficient, Wada et al reported no significant differences in totalcoronal laxity arcs between those with intact, partially ruptured ormissing cruciate ligaments [14]. The difference between our laterallaxity and that of Jenny is non-significant. On the medial side, asignificant difference is demonstrated (p<0.0001) with a mean differenceof −1.7° (95% CI: −3.1° to −0.3°). This difference in medial laxity inour study compared with Jenny's is likely below or at the margin ofclinical significance. Our laxity values are increased compared to theirstudy, thus we found no evidence to suggest contractures of the coronaltissue in 90° flexion in the OA knee at the time of TKA. We also found asmall)(−0.9° though statistically significant (p<0.001) differencebetween medial and lateral laxity in 90° of flexion. This finding isexpected, other studies examining medial-lateral laxity in the healthyknee in 70°-90° of flexion have all found greater lateral than mediallaxity. The statistical significance of these differences varied betweenstudies [7-10]. Therefore it would appear that in 90° flexion whencomparing the normal and OA knee there is relatively little realdisturbance in the coronal plane soft tissue envelope and that laxityrather than contracture is the predominant effect of osteoarthritis in90° of flexion. This is in keeping with the findings of studies at 20°of flexion comparing healthy and OA knee coronal movement arcs[14,15,18].

Literature regarding coronal plane laxity arcs at 90° of knee flexion islimited. Siston et al have published a figure of 3.1°+/−1.8° for anaverage coronal laxity arc in 90° of flexion in a series of 24 malevarus and valgus knees without severe deformity at the time of TKA [20].Ghosh et al demonstrated increasing laxity with increasing angles ofknee flexion in their cadaveric study of eight knees and a laxity arc of15° at 90° of flexion [16]. Comparison of these studies is difficultgiven different testing regimes and normal cadaveric knees compared toend stage OA knees [16,20]. We have demonstrated a laxity arc at 90° ofknee flexion of 8.5°+/−3.0° (Range: 3°-16°). Our series did not show anysignificant change in the total coronal plane laxity arc between maximumextension and 90°. However, in extension there was a greater lateralcontribution to the arc compared to flexion. The lateral tissues showeda mean decrease in laxity of 1.5° and the medial tissues a mean increaseof 2.2° between maximum extension and 90° of flexion.

These findings have practical value in TKA given the importance of thesoft tissue envelope and articular surface geometry in driving passiveknee kinematics [25, 26]. In flexion, a total coronal arc ofapproximately 8.5° is an appropriate goal. Releases that affect theflexion gap should be minimised or avoided during the approach for TKAgiven the lack of evidence of contractures of the coronal soft tissuesin this series. Both gap balancing and measured resection TKA techniqueshave demonstrated good outcomes and each has acknowledged advantages anddisadvantages [27]. Our findings have implications for both thesetechniques.

In a gap-balanced technique, careful consideration should be given tothe inherent flexion laxity. We have shown a small mean medial-lateraldifference of 1.5°+/−1.1° and 91.6% of knees in our study had amedial-lateral difference of <2.5°. In addition, we have demonstratedlateral laxity consistent with healthy knees and medial laxity increasedcompared to healthy knees. Therefore, to achieve absolute balance andthe rectangular gaps sought by the gap-balanced technique, furtherrelease of the already abnormal medial tissue or alterations of femoralimplant rotation would be required to achieve this goal. Absolute mediallateral balance is not typical of the healthy knee in 90° of flexion[7-10]. It may be better to accept the inherent laxity of the knee giventhe close symmetry present in most patients. However, this would requirefurther study given the optimal laxity post TKA in the peri-articularsoft tissue envelope remains undetermined. We found a degree ofinter-subject variability in coronal plane laxity with a maximummedial-lateral difference in 90° of flexion in our study of 8°. Somepatients may not be suitable for a gap balancing technique.

For a measured resection technique, if bony cuts have accuratelyreproduced the patient's anatomy then balance in flexion should besubstantially present. If large releases are required or significantimbalance is present then the accuracy of the bony resections should beassessed given the soft tissue tensions we have demonstrated in oursubjects in 90° of flexion.

We would acknowledge possible limitations of our study. The patientgroup was predominantly Caucasian and their selection for TKA may not beindicative of other surgical series. However, the similarity of laxityand deformity parameters between our study and the literature examiningthe OA knee would suggest our patient group is consistent with otherstudies [9,12,14,15,18,20]. Manual stress testing was performed toassess coronal plane laxity. Whilst an experienced surgeon performedthis, some degree of variability in the forces exerted during testingwould be expected. Mitigating this is the fact that ligaments areviscoelastic structures and measurements were recorded at maximaldisplacement. Therefore, measurements were recorded on the plateau ofthe tension/length curve minimising the effects of any variability inforce [19]. Our testing methods are consistent with other in-vivostudies [5,9] It is not possible in an in-vivo setting to utilise thesame invasive techniques with multiple controlled parameters andmeasurements that are utilised in cadaver studies given the ethicalimplications of prolonging surgery particularly in relation to infectionrisk. Consistent with prior studies, measurements were undertaken withminimal soft tissue disturbance designed to preserve the soft tissueenvelope [5,9]. It is also possible to take a pragmatic view of ourstudy given that our laxities were measured at a point in the surgicalexposure less than that required to complete the operation. Therefore,the measured laxities would approximate those present at a point nearthe commencement of surgery and thus our results are useful for planningsurgery from this initial point even if they do not represent the puremeasure of medial and lateral laxity in the in-vivo setting of the endstage OA knee. This study did not specifically isolate the influence ofextrinsic factors such as osteophytes both posteriorly and aroundcoronal plane tissues. We do not expect these factors would havesignificantly influenced coronal plane laxity at 90°.

CONCLUSIONS

We have defined the coronal plane laxity of a large group of patientswith end stage varus OA at the time of TKA in 90° of flexion. Our datawould suggest that the alteration between the laxity of the OA knee andits native starting point in 90° of flexion is likely to be relativelysmall in knees with a varus deformity of up to 15°. This has importantimplications for both gap balancing and measured resection TKAtechniques. Careful attention to initial laxity parameters in flexionmay help drive more individualised coronal plane soft tissue laxity andavoid unnecessary releases or distortion of the coronal soft tissueenvelope. It was beyond the scope of this paper but we have data on thepost TKA soft tissue envelope and are currently following these subjectsto examine the effect of TKA on the soft tissue envelope and theinfluence soft tissue laxity on the outcomes of TKA.

TABLE 12 Patient demographic details. Mean +/− SD Range Age (years) 64.1+/− 7.2 49-81 Sex 35 Male, 30 Female Body Mass Index (BMI) 32.3 +/− 5.420.2-52.2 FFD (degrees)  5.1 +/− 5.4  20-8.5 Varus (degrees) −7.9 +/−3.1  −15-−0.5

TABLE 13 Coronal plane laxity and total arcs of movement at maximumextension, 20°, 90° of flexion measured in degrees. Flexion Mediallaxity (°) Lateral laxity (°) Laxity arc (°) Max extension 1.6 +/− 1.1(0-4) 6.2 +/− 2.7 (0-12)  7.8 +/− 2.4 (0-14) 20° 3.0 +/− 1.7 (0-7.5) 7.0+/− 2.9 (2-15) 10.0 +/− 2.4 (6-16) 90° 3.8 +/− 1.4 (1-7.5) 4.7 +/− 2.0(1-12)  8.5 +/− 3.0 (3-16) Values are expressed as mean +/− SD withrange in brackets.

REFERENCES

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1. A method of designing a patient-specific surgical device forperforming knee surgery on the patient including the steps of: (i)obtaining patient-specific anatomical data of a patient's limb inflexion and/or extension; (ii) pre-processing and/or converting thepatient-specific anatomical data to form a patient-specific threedimensional model of the limb; (iii) determining a first alignment axisfrom a plurality of first anatomical indicators on the patient-specificthree dimensional model of the limb in extension; (iv) determining asecond alignment axis from a plurality of second anatomical indicatorson the patient-specific three dimensional model of the limb in flexion;(v) determining a flexion axis from a plurality of third anatomicalindicators on the patient-specific three-dimensional model of the limbin flexion; (vi) determining a joint line from a plurality of fourthanatomical indicators, wherein the fourth anatomical indicators areassociated with a patient's tibia and at least one of the firstanatomical indicators, the second anatomical indicators, and the thirdanatomical indicators are associated with a patient's femur; (vii)designing the patient-specific surgical device based at least partly onthe determined first and/or second alignment axes so that thepatient-specific surgical device is adapted to at least partly align afemur and/or a tibia of said limb with the first and/or second alignmentaxes; and (viii) creating the patient-specific surgical device based onthe design of the patient-specific surgical device.
 2. The method ofclaim 1, wherein the plurality of first anatomical indicators areselected from the group consisting of a central portion of a femoralhead, a central portion of a proximal femoral shaft, an intramedullarycanal insertion point, a deepest portion of a trochlear groove, acentral portion of an intercondylar notch, a central portion of a lineextending between medial and lateral tibial spines, a central portion ofa talus, a central portion of a distal tibial shaft and an anteriorcruciate ligament tibial attachment point.
 3. The method of claim 1,wherein the first alignment axis is or comprises a tibial mechanicalaxis, a femoral mechanical axis and/or a lower limb mechanical axis oran axis substantially parallel thereto.
 4. The method of claim 3,further including the step of rotating the tibia of the extended limb ina coronal plane relative to the femur, such that the tibial mechanicalaxis, the femoral mechanical axis and/or the lower limb mechanical axisare substantially parallel to the first alignment axis.
 5. The method ofclaim 1, wherein the plurality of second anatomical indicators is orcomprises: (i) a femoral anteroposterior axis and/or a tibialanteroposterior axis and the second alignment axis is substantiallyparallel thereto; and/or (ii) a transepicondylar axis (TEA) and/or aposterior condylar axis and the second alignment axis is substantiallyperpendicular thereto.
 6. The method of claim 1, wherein determining aflexion axis from a plurality of third anatomical indicators on thepatient-specific three-dimensional model of the limb, comprises rotatingthe tibia relative to the femur about the flexion axis so as tosubstantially match the degree of flexion that exists between the femurand tibia in patient-specific anatomical data of the limb in flexion. 7.The method of claim 6, wherein the plurality of third anatomicalindicators are or comprise a lateral condyle arc centre and a medialcondyle arc centre of the femur, such that the flexion axis extendstherebetween.
 8. The method of claim 6, wherein the plurality of secondanatomical indicators is or comprises the flexion axis and the secondalignment axis is substantially perpendicular thereto.
 9. The method ofclaim 1, wherein the plurality of fourth anatomical indicators areselected from the group consisting of a proximal portion of a medialtibial plateau, a proximal portion of a lateral tibial plateau, acentral portion of a lateral meniscus and a central portion of a medialmeniscus.
 10. The method of claim 1, further comprising the step ofdetermining a distal resection plane of the femur when the knee is inextension from at least partly the joint line of the knee, the firstalignment axis and/or a first dimension of a femoral prosthesis to befitted on said femur.
 11. The method of claim 10, wherein the distalfemoral resection plane is substantially parallel to the joint line ofthe knee and/or is substantially perpendicular to the first alignmentaxis.
 12. The method of claim 10, wherein the distance between the jointline and the distal resection plane are substantially equal to the firstdimension of the femoral prosthesis.
 13. The method of claim 10, whereinthe distance between the joint line and the distal resection plane isabout 0.5 mm to about 1.5 mm greater than the first dimension of thefemoral prosthesis.
 14. The method of claim 10, further comprising thestep of determining a proximal resection plane of the tibia when theknee is in extension from at least partly the distal femoral resectionplane, the first dimension of the femoral prosthesis to be fitted onsaid femur, a first dimension of a tibial prosthesis to be fitted onsaid tibia and the joint line.
 15. The method of claim 14, wherein theproximal tibial resection plane is: (i) substantially parallel to thejoint line of the knee in extension; (ii) substantially perpendicular tothe first alignment axis; and/or (iii) substantially parallel to thedistal resection plane of the knee in extension, when viewed in acoronal and/or sagittal plane of the limb.
 16. The method of claim 14,wherein the proximal resection plane is at an angle of (i) about 0.5degrees to about 15 degrees relative to the joint line and/or the distalfemoral resection plane; and/or (ii) about 75 degrees to about 89.5degrees relative to the first alignment axis, when viewed in a sagittalplane of the limb.
 17. The method of claim 14, wherein the distancebetween the distal femoral resection plane and the proximal tibialresection plane are substantially equal to the sum of the firstdimension of the femoral prosthesis and the first dimension of thetibial prosthesis.
 18. The method of claim 14, wherein the distancebetween the distal resection plane and the proximal resection plane isabout 0.5 mm to about 2.5 mm greater than the sum of the first dimensionof the femoral prosthesis and the first dimension of the tibialprosthesis.
 19. The method of claim 14, further including the step ofrotating the tibia of the flexed limb in a coronal plane relative to thefemur, such that the proximal resection plane is substantiallyperpendicular to the second alignment axis.
 20. The method of claim 14,further comprising the step of determining a posterior resection planeof the femur when the knee is in flexion from at least partly the secondalignment axis, the proximal tibial resection plane, the first dimensionof the tibial prosthesis to be fitted on the tibia and/or a seconddimension of the femoral prosthesis to be fitted on the femur.
 21. Themethod of claim 20, wherein the posterior resection plane issubstantially perpendicular to the second alignment axis and/or issubstantially parallel to the proximal tibial resection plane whenviewed in a coronal and/or sagittal plane of the limb.
 22. The method ofclaim 20, wherein the proximal resection plane is at an angle of: (i)about 0.5 degrees to about 15 degrees relative to the posteriorresection plane; and/or (ii) about 75 degrees to about 89.5 degreesrelative to the second alignment axis, when viewed in a sagittal planeof the limb.
 23. The method of claim 20, wherein the distance betweenthe posterior resection plane and the proximal resection plane issubstantially equal to the sum of the first dimension of the tibialprosthesis and the second dimension of the femoral prosthesis.
 24. Themethod of claim 20, wherein the distance between the posterior resectionplane and the proximal resection plane is about 0.5 mm to about 2.5 mmgreater than the sum of the first dimension of the tibial prosthesis andthe second dimension of the femoral prosthesis.
 25. The method of claim20, wherein the distal resection plane defines a distal femoral cutthickness and positioning, the proximal resection plane defines aproximal tibial cut thickness and positioning and/or the posteriorresection plane defines a posterior femoral cut thickness andpositioning such that a post-resection gap from said tibia to said femuris approximately equal in extension and in flexion of the knee.
 26. Themethod of claim 20, further comprising the step of determining aposition of a plurality of guide apertures in the patient-specificsurgical device for indicating or facilitating positioning of aresection member on the femur and/or tibia, wherein the resection membercomprises a plurality of resection apertures for guiding a resectiontool along the distal resection plane, the proximal resection plane, theposterior resection plane and/or an anterior resection plane of thefemur.
 27. The method of claim 26, wherein the guide apertures indicateor facilitate positioning of (i) a distal resection member on the femur;(ii) a proximal resection member on the tibia; and/or (iii) ananteroposterior resection member on the femur.
 28. The method of claim20, further comprising the step of determining a position of a pluralityof resection apertures in the patient-specific surgical device forguiding a resection tool along the distal resection plane, the proximalresection plane, the posterior resection plane and/or an anteriorresection plane of the femur.
 29. The method of claim 1, wherein thepatient-specific three-dimensional model of the limb is created usingboth soft tissue and bony tissue data from the patient-specificanatomical data.
 30. The method of claim 1, wherein the patient-specificsurgical device comprises a spacer for insertion between the femur andtibia to facilitate at least partly return of the knee to an appropriatealignment with the first alignment axis and/or the second alignmentaxis.
 31. The method of claim 1, wherein the patient-specific surgicaldevice facilitates at least partly return of the knee to an appropriateand/or balanced soft tissue tension when in extension and/or flexion;and wherein: for appropriate soft tissue tension, medial and/or lateralsoft tissue laxity of the knee in flexion and/or extension is about 1°to about 7.0°; and for balanced soft tissue tension, the differencebetween medial and lateral soft tissue laxity of the knee is or lessthan about 5°.